I haven't finished building my subs, so no measurements yet (last pair of FTW21's should arrive mid week), but based upon volume displacement capability comparison with the SI HT18D-2, here's how I manage....
A SI HT18D-2 has 5405cc of volume displacement; Sd = 1150cm2, Xmax of 23.5mm (1150 x 2.35 x 2 = 5405).
I have 4 FTW21, Sd = 1640.3, Xmax = 34mm for a total Vd of 44616cc. I also have 4 LAB12 and 4 (old) Adire Shivas with a Vd of 5269.7cc and 6118.3cc respectively for a total of 56000cc of swept displacement.
56000/5405 = 10.36 which I'll call as my SI equivalency.
Room size is 1830ft3, so 1830/10.36 = 176.6 which is my HT ULF score.
All drivers will be in sealed dual opposed with a Q of 0.707 or lower and I have a little under 19kW to power them all. Update, I just re simmed the FTW21's sealed in 220L ea (which is how I've designed them) and got a Qts of 0.577, almost the same as the SI in the data-bass test.
Here are the two compared in terms of frequency response with input power adjusted so they are almost the same at 100Hz (SI is 0.3dB higher) so the difference between then will be about 0.5dB at 10Hz, all being equivalent. Close enough for guv'mint work.
I don't have anywhere convenient to do CEA tests like Josh and I doubt I'll be able to test at full power when finished so this will have to do.