I read this thread with great interest as getting subs to line up properly in my systems has been the bane of my existence for years. In all honesty, there is no better treatise on this than Soundoctor Barry Ober's excellent whitepaper cited in the OP. It's a dense but masterful summary of the state of the art on the topic. There are a few things however, on which I would like to comment.
First, as Barry says, it's critical to understand that in most systems, the only way to get the subs, which are generally placed behind the main speakers, to time align with the mains, is to retard the time arrival of the mains. And that requires DSP. It's really that simple. Barry cites that in some rare cases one might move the subs forward 9 or 10 feet to achieve this but while that might be possible in some large studio venues, it is an almost impossible request for the home listening room.
Second, Barry says that the next best solution is to use a crossover such as the excellent JL Audio CR-1 and accept the fact that the best one can achieve is to have a situation whereby the bass and mains are phase aligned (by judicious use of the phase control in the crossover) but one needs to accept that at the crossover point, the subs will always be approximately 1 cycle behind in comparison to the the mains. Most importantly, he strongly suggests that such an error will not be objectionable in most systems if the crossover is optimally set-up. That comment is one that almost nobody else makes, but to me, it makes Barry a very wise man indeed.
I have spent many years months trying both of these proposed solutions. I have used DSP systems as well as the superb JL Audio CR-1 crossover. Much to the dismay of some audiophiles, I'm sorry to report there is no "best" or "right" answer. There are always tradeoffs. Some of these have been discussed in depth previously.
http://www.whatsbestforum.com/showthread.php?520-DSP-one-person-s-experience
http://www.whatsbestforum.com/showt...T-digital-crossover&highlight=spectral+jersey
To make matters worse I am currently using a system in which the JL Gotham subs are integrated with my Wilson Alexandria S-2's in the manner that Barry does not recommend which is namely, running the mains full range and augmenting them with subs at a very low frequency (i.e. 30 Hz) though a second set of pre-amp outputs and the integral crossover on the Gothams. All I can tell you about that is that I have tried every other method Barry has suggested, and have come away thinking that my current method is the one I prefer because it offer the best balance of sonic merits/liabilities. It would be a lengthy discourse indeed to go into all the reasons why I have chosen that approach except to cite the obvious- no other method I have tried allows for the same degree of pristine signal from preamp to the mains. As good as the CR-1 is, there is still a sonic degradation through the high pass that is noticeable in comparison to "bypass" which can be done at the push of a button. And for me, once heard, there is no going back. Put another way, my system ain't perfect from a time-alignment perspective as assessed by impulse measurement, but it offers a satisfactory sonic experience nonetheless.
I cannot emphasize enough the wisdom of Barry's final comments in his white paper which is worth repeating here:
"Even if you CAN'T get the timing of your sub to match your mains as closely as it can be done, there IS a saving grace.... Notice that humans actually LIKE the fattening up of the bass loudness envelope in time. Therefore even IF your sub is 12 msec late, and you are one wavelength off, as long as you get that delayed wavelength to line up with the bass coming out of your mains, your frequency response will be pretty good and you won't have any awful objections, again, assuming you get as much else right as possible"
What Barry is saying is something audiophiles have known for years. PERFECT can be the enemy of GOOD. When it comes to adding subs, it's always prudent to keep that in mind. Let your ears be your guide.