There are many things to consider in chasing the first 2 octaves.
A sealed sub is the way to go, short of the TRW.
The Marchand Bassis is an excellent piece of hardware in that it allows you to instantly transform your signal to any driver(s). This allows you to experiment with different drivers and/or change drivers as time marches on with little effort, rather than laboring to extract the best performance from a driver compliment in a particular box and room.
Dr. Phil builds extremely accurate, high quality analog hardware and it's roll off is nearly absolutely predictable through the amount of boost, Fs and Qs settings, which allows for some compensation of electronics roll off in the extreme low end.
Use of box size with the Bassis affords excursion protection without sacrificing desired system Q and Fb. This means that if you carefully consider the combination of L/T signal mod, box size, driver and amplifier power, you won't need a subsonic HP filter.
THD at single digits, using traditional cone drivers is a factor that won't show up in the models. There can easily be at or near 100% THD at maximum output when reproducing single digit Hz at high levels. Since the 2nd and 3rd harmonics, where most of the THD will be generated, is subsonic (example: 5Hz...the 2nd harmonic is 10Hz and the 3rd is 15Hz, both inaudible frequencies), there is a free lunch effect.
This adds impact without adding any displeasing audible effect. Remember that these engineers mix by listening to the results, not by staring at meters. We've all been taught that THD is a horrible thing, but in the first 2 octaves (from the Dolby spec of 3Hz to 12Hz.), I don't think there are many experienced comments to be had on the subject.
Soft clipping of an amp like the QSC is not a terribly bad thing at single digit playback. These amplifiers are designed to limit gross clipping and I believe that if there were no clip indicator LEDs to alert you to a soft clip at single digits, no one here could tell it ever happened.
Yes, the purist in us would argue that THD is bad and soft clipping alters the signal, but if you're really gonna push your system beyond it's limits every time you watch a movie or listen to music, you simply need more system.
All of these discussions seem to center around maximums. Maximum excursion. Maximum dBSPL. Maximum Vd. Etc., etc.
What the reality is is that you'll listen at a level that's comfortable for you, in your room, with your system. That will typically be nowhere near the system's maximum capability.
This is why the Bassis, with a sealed system is ideal. If you're seriously considering the Bassis dialed to a 22dB boost, that would mean full signal boost at around 14Hz and below. If you are listening with average peaks at 110dB from the low end system, that's around 125 watts at 50Hz and 2,000 watts at 14Hz and below. If you shoot for 3dB of amp headroom you'll need 4,000 watts, which, as the OP correctly states, requires big current from a single AC outlet.
This means that the low end gets the power it needs to keep up with the upper bass range without the need to run the sub extremely hot, which is just a dog chasing it's tail, IMO.
When you factor in boundary gain and room gain, I believe that you'l find the boost figure will end up being dialed down from 22dB, depending on your average listening levels and the placement of the sub in the room. This again is another good reason to have a Bassis. You can simply choose a lesser boost position on the boost dial. No cussing that you shoulda or hafta, after the fact.
I've had some 50 drivers in my system in configs of 1X10" through 8X15", in boxes ranging from .7 cubes to 7.5 cubes, powered by 400 watts to 7,200 watts. I use 2 modded Bassis Bi-Quads and a custom built analog Rebase preamp, so I've been able to experiment and measure many, mnay combinations of low end capability.
Chasing the bottom of the bottom is a PITA, but it's rewarding, and a trip from which there is no return.:clap:
Bosso