Yes, that's what I'm talking about! Thank you for posting that link. The Purist website looks inactive. From the photos the build quality looks chintzy.
If ML itself makes a SuperNeolith with stackable, matching BalancedForce 215s then I won't have to!
Yes, it is a risk to do it myself. I would much prefer a fully-integrated four tower solution made by the same designer (e.g., Dragon, Pendragon, MM7). The Neolith's 12" driver working up to 400 Hz might allow the Neolith slightly more oomph and cone excursion impact in that range than even the Dragon (100 Hz or lower crossover) or the Pendragon (200 Hz crossover).
On the other hand, if the low pass filter of a custom subwoofer covers a wide range and the crossover controls are flexible and phase is adjustable I do not see why custom subwoofer towers could not be made to integrate with the Neolith, especially since the 15" driver in the Neolith can be attenuated by -2, -4 and -8 dB.
I could attenuate the 15" driver in the Neolith and allow the towers to work up to 200 Hz and overlap with the 12" and the 15" drivers (which might ameliorate User211's sense that the bass sounds likes it is coming from the bottom third of the speaker), or I could use the conventional formula of bringing the subwoofer towers in under the 15" drivers with a crossover of 40 Hz or so.
The question then is: which subwoofer towers?
A) custom Mark Seaton (dual-opposed 15" drivers firing front and back, designed for stacking, very flexible filters and controls, class D amplification),
B) stacked ML 212s (dual-opposed 12" drivers firing left and right, highest low-pass setting is 80 Hz, class D amplification, awkward to stack since they are not designed for stacking),
C) stacked Rel Gibraltar G-1s (single front-firing 12" driver, flexible controls, class AB amplification, designed for stacking), or
D) a pair of Wilson Thor's Hammers with Watch Controller driven by VTL MB-450s (two front firing 15" drivers per side, very flexible controls, passive so I can select the amplifier) (DDK's preference)
I have been corresponding with Mark about using class AB amplification instead of class D, but that adds a lot of weight and complexity. And we don't know that some generic AB amplifier necessarily sounds better than a well-designed class D. (Driving Mark's dual-opposed subwoofer modules with banks of VTL MB-450s doesn't seem very practical.)
The final question is, and this circles back to User211's concern that this is sort of an experiment (which is why MikeL likes one-company solutions -- and I agree) is that it would be helpful to know which crossover regime is better: the higher-crossover/overlap solution or the conventional low crossover. For the higher-crossover/overlap at 100 to 200 Hz I might prefer a stack of 12" drivers, whereas for the conventional crossover at about 40 Hz I might prefer a stack of 15" drivers.
ML used an open baffle for the mid-bass towers on the Statement E-2. Dragon, ML and Mark Seaton use a dual-opposed (bipole) design. Pendragon is front-firing.