Bass Stacks

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
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Very interesting indeed! Do you recall the old Celestion SL6000 dipole subs which went underneath the SL600si bookshelf speakers to make them more full range? https://www.stereophile.com/floorloudspeakers/892/index.html

View attachment 33411

Any thoughts on this design?

I've seen a few designs of this type, and for most cases where I envision a dipole subwoofer making sense, I think a more straight forward U-Baffle or H-Baffle, akin to the old Audio Artistry active systems, is a more efficient and effective way to go. I fondly remember the first time I heard a Dvorak system and later the Beethoven system set up properly. The drive units available at that time are considered quite modest by today's standards, and our understanding of the modeling and room interaction are much more advanced than 20 years ago when those were brought to market. Dipole subwoofers probably fall well into the niche within a niche realm, but they can be easy to get sounding great in a real room. The room interaction and radiation do a great job complimenting panels or even horns if you have enough capability to keep them linear.
 

Ron Resnick

Site Co-Owner, Administrator
Jan 24, 2015
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That all sounds very exciting, Mark! Dipole/open baffle is very interesting and resin/quartz seems like a great material for a subwoofer cabinet.
 

LL21

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Dec 26, 2010
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I've seen a few designs of this type, and for most cases where I envision a dipole subwoofer making sense, I think a more straight forward U-Baffle or H-Baffle, akin to the old Audio Artistry active systems, is a more efficient and effective way to go. I fondly remember the first time I heard a Dvorak system and later the Beethoven system set up properly. The drive units available at that time are considered quite modest by today's standards, and our understanding of the modeling and room interaction are much more advanced than 20 years ago when those were brought to market. Dipole subwoofers probably fall well into the niche within a niche realm, but they can be easy to get sounding great in a real room. The room interaction and radiation do a great job complimenting panels or even horns if you have enough capability to keep them linear.

Thanks! What is a U or H Baffle? The Audio Artistry 4-tower speakers look interesting as the 2 woofer towers look more room friendly size-size.
 

sbo6

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In trying to assist me in sub placement, Mark pointed me to REW which is free measuring software that will run on a mac or a PC. I already have measuring software but in the software there is a subwoofer placement simulation mode, which, as it turns out, is VERY accurate - not perfect but more than accurate enough. It allows you to place up to 4 subs (6 if you make the mains subs) anywhere in the room, including stacking them.

Highly recommended as a staring point and it beats the heck out of moving subs around and measuring them - which is still, in my mind, eventually necessary to validate your findings. Again, highly recommended.

REW is a fantastic free tool no doubt. For sub simulation I think 1 caviat is that it only allows measurement entries for a rectangle so no odd shaped rooms allowed unfortunately.
 

audioguy

WBF Founding Member
Apr 20, 2010
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REW is a fantastic free tool no doubt. For sub simulation I think 1 caviat is that it only allows measurement entries for a rectangle so no odd shaped rooms allowed unfortunately.

Correct. I know that eliminates a lot of folks who have their system in a non-dedicated space - and even some who do have a dedicated (but non-rectangular) space. I've been fortunate (or maybe determined) to have dedicated and rectangular spaces since 1991 - and in all homes following.

But for those that do have sealed rectangular spaces, it is an amazing and helpful tool.
 

infinitely baffled

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We already use and I work with some builders of exceptionally low distortion woofers. These do not always provide the highest level of output for their size, but are highly capable with typical music demands, and multiple units can make for highly impressive movie playback for those pulling double duty or for the audiophiles wanting something even more refined in their home theater system. I've regularly demonstrated over the past 12 years how we can reliably deliver 6-12Hz in-room extension with the sealed subwoofers I've offered, and even done a few ported and bandpass subs tuned into the 9-12Hz range. Delivering such extension while maintaining the upper bass articulation can be a trick, and is not automatic, but very important.

I have a few concepts I'll pursue, but will start with a couple sealed solutions with front end DSP and the latest ICEpower amplifiers which are quiet and very well behaved for subwoofer use. I see a definite need for some competent dipole/open baffle woofers, and some examples of each we can do with external analog electronics if desired.

One of the materials I'll be experimenting with will be quartz, as the manufactured nature makes is consistent and super stiff while not needing exotic finishing beyond polishing. We'll see what gets finalized first.

Thank you for you compliments and enthusiasm,

Dipole subwoofers mmmmmm :cool:
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
381
141
390
47
Chicago, IL
www.seatonsound.net
Dipole subwoofers mmmmmm :cool:

In your room with the lossy windows, it might be a useful, but you would need a LOT of them for your room size, tastes, and range you are crossing over. I still think your best bet is a Trinnov and a 3-4 tall stack of my F18s left & right. With some measurements and tinkering with delay settings, my other hunch is that you could use 4 sub locations with 2 in front near the speakers near your current stacks but wider, and then another pair at the rear wall which better delivers the deep bass. The key would getting great sound from this will be delaying the rear subwoofers such that they "hit" the listening position after the front pair and speakers. Setting the delay is easy, finding a suitable delay that arrives later and provides a good summation is where some experimenting comes in.

The precedence effect and/or Hass Effect is why this can work. In effect the rear pair are filling in the gaps of the front subs, but adjusted not to call attention to themselves. So long as you have some distance between the rear subs and the listener, this can work very well, although some wouldn't call it as "purist" a solution. Of course a purist solution would spin your room around and add many more acoustic treatments. This is a way to get similar result without compromising the aesthetic you are after.
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
381
141
390
47
Chicago, IL
www.seatonsound.net
Thanks! What is a U or H Baffle? The Audio Artistry 4-tower speakers look interesting as the 2 woofer towers look more room friendly size-size.

If you look at the open baffle speakers like the base models from Spacial, Emerald Physics, and a few others, most are a driver on a baffle of some width. The width sets the low frequency corner below which the front and rear output cancels itself at a rate of 6dB/octave. The wider the width, the longer the path from back to front, and lower in frequency they start canceling. The benefit is no air spring in a box to fight, and the output cancels in the plane of the baffle (90 deg to the aim of the speaker). The biggest trade off can be efficiency at low frequencies and the required volume-displacement, or how much air the woofers have to move to produce a given output. The narrower the baffle, the greater the displacement required at low frequencies.

For wide bandwidth speakers, the simpler flat baffle reduces any concerns of reflections at higher frequencies. For lower frequency use as a dipole subwoofer, we can put ends on the baffle to extend forward and back, such that the overhead view looks like an H with the woofer on the horizontal flat piece. This increases the path distance between the front and rear side of the woofers, moving the frequency where each side begins to cancel lower, akin to widening the baffle. In short, we can extend the baffle forward and back to increase the low frequency efficiency.

A U-Baffle has a similar concept, but the woofer is at the bottom of the U in an overhead view. This shifts the cancellation point from the sides to the rear of the speaker, creating a cardioid pattern. Rather than using an open back, Martin Logan uses a 2nd woofer on the back of some of their panel speakers to create a similar cancellation where you see what they brand as "Force Forward" technology. The U baffle works with just one woofer, and has a related interaction with room modes like a dipole woofer. Creative placement can counteract length modes of the room, where a dipole doesn't directly excite width modes, and has a different interaction with the length room modes than a sealed woofer.

Both a benefit and detriment of dipole and open cardioid designs is that they do not present the classic low frequency room gain I take advantage of in my sealed designs. By driving both positive and negative energy into the room, you don't see the overall summing of the lowest frequency reflections due to the overall volume and dimensions of the room. This does help keep decay times under control, but also requires much more air movement from the woofers to produce the same level. While the rear and side walls can be used to extend the response in some rooms, overall this means the woofer has to have the outdoor/anechoic low frequency extension rather than taking advantage of the free gain most rooms provide. By comparison, in many enclosed listening rooms a sealed subwoofer with a natural roll off point in the 16-22Hz range can provide real extension at the listener to below 10Hz.
 

LL21

Well-Known Member
Dec 26, 2010
14,376
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If you look at the open baffle speakers like the base models from Spacial, Emerald Physics, and a few others, most are a driver on a baffle of some width. The width sets the low frequency corner below which the front and rear output cancels itself at a rate of 6dB/octave. The wider the width, the longer the path from back to front, and lower in frequency they start canceling. The benefit is no air spring in a box to fight, and the output cancels in the plane of the baffle (90 deg to the aim of the speaker). The biggest trade off can be efficiency at low frequencies and the required volume-displacement, or how much air the woofers have to move to produce a given output. The narrower the baffle, the greater the displacement required at low frequencies.

For wide bandwidth speakers, the simpler flat baffle reduces any concerns of reflections at higher frequencies. For lower frequency use as a dipole subwoofer, we can put ends on the baffle to extend forward and back, such that the overhead view looks like an H with the woofer on the horizontal flat piece. This increases the path distance between the front and rear side of the woofers, moving the frequency where each side begins to cancel lower, akin to widening the baffle. In short, we can extend the baffle forward and back to increase the low frequency efficiency.

A U-Baffle has a similar concept, but the woofer is at the bottom of the U in an overhead view. This shifts the cancellation point from the sides to the rear of the speaker, creating a cardioid pattern. Rather than using an open back, Martin Logan uses a 2nd woofer on the back of some of their panel speakers to create a similar cancellation where you see what they brand as "Force Forward" technology. The U baffle works with just one woofer, and has a related interaction with room modes like a dipole woofer. Creative placement can counteract length modes of the room, where a dipole doesn't directly excite width modes, and has a different interaction with the length room modes than a sealed woofer.

Both a benefit and detriment of dipole and open cardioid designs is that they do not present the classic low frequency room gain I take advantage of in my sealed designs. By driving both positive and negative energy into the room, you don't see the overall summing of the lowest frequency reflections due to the overall volume and dimensions of the room. This does help keep decay times under control, but also requires much more air movement from the woofers to produce the same level. While the rear and side walls can be used to extend the response in some rooms, overall this means the woofer has to have the outdoor/anechoic low frequency extension rather than taking advantage of the free gain most rooms provide. By comparison, in many enclosed listening rooms a sealed subwoofer with a natural roll off point in the 16-22Hz range can provide real extension at the listener to below 10Hz.

Fascinating...i'll probably need to read this a few times to get through and pseudo understand it...but thank you. That starts to make some sense. I guess the key is (as usual) in execution...as the designs that use room gain need that part controlled...while those that done have their own inherent stresses to generate comparable power...which also comes with risk of distortion.
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
381
141
390
47
Chicago, IL
www.seatonsound.net
Fascinating...i'll probably need to read this a few times to get through and pseudo understand it...but thank you. That starts to make some sense. I guess the key is (as usual) in execution...as the designs that use room gain need that part controlled...while those that don't have their own inherent stresses to generate comparable power...which also comes with risk of distortion.

I'd suggest some quick perusing of some open baffle DIY websites that describe the operation, radiation pattern, and low frequency roll off if you find it of interest. While a bit math heavy, Sigfried Linkwitz's website, linkwitzlab.com, is another good resource about dipole speakers. Sigfried Linkwitz was the designer of all the Audio Artistry speakers from long ago which were well ahead of their time with external, active crossovers, and much more attention to off axis energy and power response than others of their day.

You are on track with the understanding, and I would also highlight the reality that a dipole woofer does not need the high motor strength or higher mass required to get bass from of a manageable size, sealed or reflex (vented or PR) enclosure since it is not driving the air spring or port. As should be obvious, drivers for dipole use do need significant volume displacement through whatever combination of cone area and excursion, so more significant excursion is generally desirable. With some creative matching of the woofer's natural resonance and the front-back cancellation due to the baffle dimensions, we can get away with a driver with less motor force than we could use in a normal box, and actually get deeper extension without EQ. This low motor force means we can get away with much lower mass voice coils to keep efficiency reasonable in the upper bass range. In the end the best results come from specifically optimized woofers that aren't very useful in any application other than an open front/back design.
 

LL21

Well-Known Member
Dec 26, 2010
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Thanks for that!!! Once again, very helpful!
 

audioguy

WBF Founding Member
Apr 20, 2010
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As you can surmise from his above posts, Mark knows A LOT about bass and subwoofers. And his products reflect that knowledge and, while not obvious here, his obsession for perfection. The drivers in his subs, while built by commercial manufacturers are all custom built to his very detailed specifications.

I have owned a lot of subs sold in "brick and mortar" stores and heard lots more. I am comfortable saying that his subwoofer products are as good as "the best", and in many cases, far, far superior --- for both music and home theater. And when it comes to "value", hard to beat. While I do recognize that "preference" plays a huge roll in the selection of our audio components, I would strongly urge anyone thinking of adding (or upgrading/changing) subs to their two channel (or home theater) system to actively consider Seaton products.

And no, I am not a paid employee. I bought my first Seaton subs about 8 years ago. He was a much smaller company then and while attending a CEDIA show in Atlanta, he came to my home to "assist" in the placement and setup of the two SubMersives I bought (they had replaced some B&M subs I owned at the time). It was then that I began to understand how smart, fastidious and good he was at his craft. He is also a really good guy. Over the last 8 years I wandered away from his bass products (because that is what folks afflicted with this [strike] addiction[/strike] hobby do on occasion), but have come back full circle and will end up with a bunch of his F18's for my room. I quickly realized there was "different" and, in the case of DIY, less expensive (turned out to be more expensive once I had to sell my DIY subs and go ack to Marks'), but not "better".

Just sayin'
 

infinitely baffled

VIP/Donor
Jul 2, 2015
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In your room with the lossy windows, it might be a useful, but you would need a LOT of them for your room size, tastes, and range you are crossing over. I still think your best bet is a Trinnov and a 3-4 tall stack of my F18s left & right. With some measurements and tinkering with delay settings, my other hunch is that you could use 4 sub locations with 2 in front near the speakers near your current stacks but wider, and then another pair at the rear wall which better delivers the deep bass. The key would getting great sound from this will be delaying the rear subwoofers such that they "hit" the listening position after the front pair and speakers. Setting the delay is easy, finding a suitable delay that arrives later and provides a good summation is where some experimenting comes in.

The precedence effect and/or Hass Effect is why this can work. In effect the rear pair are filling in the gaps of the front subs, but adjusted not to call attention to themselves. So long as you have some distance between the rear subs and the listener, this can work very well, although some wouldn't call it as "purist" a solution. Of course a purist solution would spin your room around and add many more acoustic treatments. This is a way to get similar result without compromising the aesthetic you are after.

Can I ask, how do your f18s compare with the Velodyne dd18+ ?
 

audioguy

WBF Founding Member
Apr 20, 2010
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Near Atlanta, GA but not too near!
Can I ask, how do your f18s compare with the Velodyne dd18+ ?

I'm not Mark but I can offer some perspective. I had Seaton SubMersives at the time this occurred. I had two DD18's (not the pluses) at the same time I had the 2 SubMersives. The ONLY place the DD18's came out on top was the built in EQ system. But for the huge difference in price, I bought an external EQ system (miniDSP) that was far superior to the one built into the DD18's and of course the total cost of the SubMersives plus EQ was far less than the Velodynes. I sold the DD18's and purchased two more SubMersives (can never have too many subs :D)

The Seaton's have much lower extension, better detail in the upper bass, far more impact in the mid bass. They blended perfectly with a number of speakers I have had. When I first got them, I had Dunlavy SC-VI's.

I know you are asking about the F18's and I now have (for another week or so) F18's and SubMersives in the same room (I am replacing the SubMersives with more F18's). The primary difference between the sonic signature of the two is that the F18's have more "oomph" (not a high end audio word) at the very bottom end. Not as a function of frequency response but just the ability to deliver cleaner output at the lowest octave. Otherwise all of the positive attributes of the SubMersive in present in the F18's but with a smaller cabinet, less expense and modular growth ability.
 

LL21

Well-Known Member
Dec 26, 2010
14,376
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I'm not Mark but I can offer some perspective. I had Seaton SubMersives at the time this occurred. I had two DD18's (not the pluses) at the same time I had the 2 SubMersives. The ONLY place the DD18's came out on top was the built in EQ system. But for the huge difference in price, I bought an external EQ system (miniDSP) that was far superior to the one built into the DD18's and of course the total cost of the SubMersives plus EQ was far less than the Velodynes. I sold the DD18's and purchased two more SubMersives (can never have too many subs :D)

The Seaton's have much lower extension, better detail in the upper bass, far more impact in the mid bass. They blended perfectly with a number of speakers I have had. When I first got them, I had Dunlavy SC-VI's.

I know you are asking about the F18's and I now have (for another week or so) F18's and SubMersives in the same room (I am replacing the SubMersives with more F18's). The primary difference between the sonic signature of the two is that the F18's have more "oomph" (not a high end audio word) at the very bottom end. Not as a function of frequency response but just the ability to deliver cleaner output at the lowest octave. Otherwise all of the positive attributes of the SubMersive in present in the F18's but with a smaller cabinet, less expense and modular growth ability.

Great stuff and thanks for that feedback. I have read nothing but great things about Seaton Subs...congrats. Infinitely Baffled - I owned both the DD18 and now the DD18+ which is much, much better than the old DD18. Putting them side by side was only of those 'not a fair fights' where, potentially inspired by the Seaton, JL Gotham and other great, great subs, Velodyne did step up their game quite a bit.

However, whether this substantial improvement (if the DD18+ was arbitrarily a 100 score, i would have scored the DD18 at closer to 75, the old DD15 at a 62 (which i have also done side by side)...is enough to beat the Seaton, i have no idea. But i DO suspect on a value basis, the Seaton does win by quite a margin.
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
381
141
390
47
Chicago, IL
www.seatonsound.net
Can I ask, how do your f18s compare with the Velodyne dd18+ ?

Hi ib,

Much of what audioguy applies, with the exception that the DD18+ did make a big upgrade to the woofer and brought back some box size, both of which greatly helped the lowest octave vs the original DD15. The new heftier woofer did compromise the upper octave a bit, and the servo is the only thing keeping the distortion in the ballpark of our F18. Overall the F18 still has a notable output advantage in a 1:1 box comparison, while my design using the single large amplifier to power multiple units means you aren't spending 3-4x as much when you want 3-4 units per side, and keeping them working happily together is as simple as it gets (a twist-locking SpeakOn umbilical between each). The F18 has very smooth extension out past 100Hz which is quite important for detail and attack, while the response and lowest frequency roll off of the F18 is much better suited to achieving 6-10Hz in-room extension. The while still extending quite low, the DD18+ is a bit overly flat outdoors to the ~15Hz range and then turns a sharp corner cutting off most energy below 14Hz. The starting frequency response of the F18 is much easier to match with a real room, especially with the LF Adjust knob which shelves the response up/down to match your room or tastes.

In the case you did test the limits of the subs, the F18 is very well composed where you are unlikely to know you are tickling the limits unless you compared to even more units for comparison. For an actual upgrade rather than just a swap, I would suggest 3 units per side, 4 if you like the 8 ft. (2.4m) tall stack. Between our direct sale pricing and the huge cost savings in the F18+/F18-Slave approach, the set of 3 are not that far from the price of a single DD18+. Obviously the big trade here lies in physically larger boxes/stack.

Overall a stack of F18s would increase headroom and overall detail while insuring effortless output. The design of the F18 will make for smoother extension below 20Hz on down to around 10Hz in room. The headroom will also be very welcome for any EQ needed to deal with the limited placement options and acoustic challenges of your room (especially with something like a Trinnov in the mix).

I do also have 2 other versions of the F18 we worked up for custom installs called the DS18-20 & DS18-12, one having a 20" x 20" face with 23.5" depth if you needed a narrower but deeper footprint, and the other having a 30" x 26" face but only 12.5" deep. For the system you've pictured and described, I would stick with our standard F18 as they also have a more sculpted cover panel framing the impressive looking woofer, and the 18" depth works better in a wider range of placement options for typical placements keeping more options open with only a 23.5" x 17.5" (597mm x 445mm) footprint each:

 

infinitely baffled

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Jul 2, 2015
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Food for thought!

Thank you very much, Mark
 

LL21

Well-Known Member
Dec 26, 2010
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infinitely baffled

VIP/Donor
Jul 2, 2015
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Scotland
Hi ib,

Much of what audioguy applies, with the exception that the DD18+ did make a big upgrade to the woofer and brought back some box size, both of which greatly helped the lowest octave vs the original DD15. The new heftier woofer did compromise the upper octave a bit, and the servo is the only thing keeping the distortion in the ballpark of our F18. Overall the F18 still has a notable output advantage in a 1:1 box comparison, while my design using the single large amplifier to power multiple units means you aren't spending 3-4x as much when you want 3-4 units per side, and keeping them working happily together is as simple as it gets (a twist-locking SpeakOn umbilical between each). The F18 has very smooth extension out past 100Hz which is quite important for detail and attack, while the response and lowest frequency roll off of the F18 is much better suited to achieving 6-10Hz in-room extension. The while still extending quite low, the DD18+ is a bit overly flat outdoors to the ~15Hz range and then turns a sharp corner cutting off most energy below 14Hz. The starting frequency response of the F18 is much easier to match with a real room, especially with the LF Adjust knob which shelves the response up/down to match your room or tastes.

In the case you did test the limits of the subs, the F18 is very well composed where you are unlikely to know you are tickling the limits unless you compared to even more units for comparison. For an actual upgrade rather than just a swap, I would suggest 3 units per side, 4 if you like the 8 ft. (2.4m) tall stack. Between our direct sale pricing and the huge cost savings in the F18+/F18-Slave approach, the set of 3 are not that far from the price of a single DD18+. Obviously the big trade here lies in physically larger boxes/stack.

Overall a stack of F18s would increase headroom and overall detail while insuring effortless output. The design of the F18 will make for smoother extension below 20Hz on down to around 10Hz in room. The headroom will also be very welcome for any EQ needed to deal with the limited placement options and acoustic challenges of your room (especially with something like a Trinnov in the mix).

I do also have 2 other versions of the F18 we worked up for custom installs called the DS18-20 & DS18-12, one having a 20" x 20" face with 23.5" depth if you needed a narrower but deeper footprint, and the other having a 30" x 26" face but only 12.5" deep. For the system you've pictured and described, I would stick with our standard F18 as they also have a more sculpted cover panel framing the impressive looking woofer, and the 18" depth works better in a wider range of placement options for typical placements keeping more options open with only a 23.5" x 17.5" (597mm x 445mm) footprint each:


Mark you have pm

The umbilical you describe is very appealing....i have two large dogs and the trailing leads round the back of the stacks are seriously inconvenient!

It adds to the feeling that this sort of an installation is an outlier for Velodyne.
 
Last edited:

infinitely baffled

VIP/Donor
Jul 2, 2015
1,259
387
340
Scotland
Hi ib,

Much of what audioguy applies, with the exception that the DD18+ did make a big upgrade to the woofer and brought back some box size, both of which greatly helped the lowest octave vs the original DD15. The new heftier woofer did compromise the upper octave a bit, and the servo is the only thing keeping the distortion in the ballpark of our F18. Overall the F18 still has a notable output advantage in a 1:1 box comparison, while my design using the single large amplifier to power multiple units means you aren't spending 3-4x as much when you want 3-4 units per side, and keeping them working happily together is as simple as it gets (a twist-locking SpeakOn umbilical between each). The F18 has very smooth extension out past 100Hz which is quite important for detail and attack, while the response and lowest frequency roll off of the F18 is much better suited to achieving 6-10Hz in-room extension. The while still extending quite low, the DD18+ is a bit overly flat outdoors to the ~15Hz range and then turns a sharp corner cutting off most energy below 14Hz. The starting frequency response of the F18 is much easier to match with a real room, especially with the LF Adjust knob which shelves the response up/down to match your room or tastes.

In the case you did test the limits of the subs, the F18 is very well composed where you are unlikely to know you are tickling the limits unless you compared to even more units for comparison. For an actual upgrade rather than just a swap, I would suggest 3 units per side, 4 if you like the 8 ft. (2.4m) tall stack. Between our direct sale pricing and the huge cost savings in the F18+/F18-Slave approach, the set of 3 are not that far from the price of a single DD18+. Obviously the big trade here lies in physically larger boxes/stack.

Overall a stack of F18s would increase headroom and overall detail while insuring effortless output. The design of the F18 will make for smoother extension below 20Hz on down to around 10Hz in room. The headroom will also be very welcome for any EQ needed to deal with the limited placement options and acoustic challenges of your room (especially with something like a Trinnov in the mix).

I do also have 2 other versions of the F18 we worked up for custom installs called the DS18-20 & DS18-12, one having a 20" x 20" face with 23.5" depth if you needed a narrower but deeper footprint, and the other having a 30" x 26" face but only 12.5" deep. For the system you've pictured and described, I would stick with our standard F18 as they also have a more sculpted cover panel framing the impressive looking woofer, and the 18" depth works better in a wider range of placement options for typical placements keeping more options open with only a 23.5" x 17.5" (597mm x 445mm)

Responding to some of your other points,
I would instinctively prefer all the subs to be active, I'm not looking to skimp on the amplification. each sub will have a dedicated mains strap, and they will collectively have their own breaker
Likewise with the various dimensions you have described, for me the only consideration is performance. So whatever gives the best results, from your whole range.

The Trinnov didn't really impress when it was here. When Nick blind played me through the various settings I consistently preferred the one where they were all off. My system has a lightness to it that was absent when the Trinnov was engaged. Nick hasnt mentioned it since, I will have to ask him what his conclusion was. On the day he put it down to the Wavac / Boulder combi sounding nice
 

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