Maybe some people think AVS is more prominent and important in the world of home theater than it really is? Is Floyd Toole a regular contributor or member here? What about Thom Holman? I believe both Henry Kloss and Wilbur Klipsch were alive and active during the existence of AVS, I don't recall them being contributors?
While there seems to be a great deal of focus on the quality of Dr. Geddes room as impacting the response, you have to understand that one of the major benefits of the multiple subs around the room is that it corrects for the problems caused by the room. Even those of us without good dedicated rooms will benefit from this, probably more so. Then again, there isn't any other way to skin this cat (Smoothness spatially and amplitude).
We should differentiate time alignment and phase here. While technically the phase adjustment is making a time adjustment, What Dr. Geddes advocates is adjusting the phase. A wave could be in phase but not in time if the time difference was large enough, many many ms's for instance. So keep in mind that when doing this, phase adjustment of the subs is very important to achieving smooth response. However, if somehow you managed to get the subs a full 10ms off, the phase adjustment wouldn't correct for this, but you could still phase align the soundwave enough for a smooth response. While it will have an impact on the smoothness of the response, I think Dr. Geddes was trying to say that you wouldn't hear it as a delay until the delay exceeds 40-50ms.
While we all are very proud of our very expensive high excursion subwoofers, keep in mind that "max spl" is a somewhat relative term, and most of these subwoofers are very inefficient, Dr. Geddes are actually quite efficient (The ulf probably isn't). An 18" or 15" B&C LF driver is capable of well in excess of 120db's from 50hz on up. If you have 2 of these, they will match or exceed plenty of systems using a similar number of TC Sounds or Whatever brand 30mm xmax subs, in that range. Where things would change I suspect is below that 50hz point. Displacement becomes pretty important at those frequencies, and I would suspect that where the high xmax subs would begin to outdo Dr. Geddes subs is in that range. But then, does it matter? With regard to the majority of the bass range (say 40-150hz), we have plenty of output and a very smooth response, and this is likely to be the most audible area to get right. How loud your "sub" bass is will probably be a matter of choice, and you can buy as many ULF's subs or build as many High xmax subs for this range as your heart desires. While I'm sure you can still have improvements from multiple ULF subs (of whatever design you choose), I don't suspect that it's as important (though shear output might).
Ok back to the quality of the sub question. If a company other than Dr. Geddes was to produce the subs using identical woofers, cabinets, and even amplifiers, they would cost a lot more than 300 dollars. They maybe don't need to (Even he will be selling them for 4-500), but they would. The cost of the drivers he uses would be in excess of 200-400 dollars for us (The 18" I think retails for 450), so these are not cheap drivers. The reason for the amps is because the high sensitivity and lower xmax means the driver will hit it's mechanical limits with a lot less power, but that doesn't mean less output. I can only guess at the particulars of Dr. Geddes alignment, but my models suggest that the 15" B&C driver in a bandpass box would produce 120db's from 50-150hz with just a few hundred watts, but would exceed xmax with any more than 300 watts. By comparison my TC driver needs 1500 watts to do the same thing. Since Reference levels dictate 115db's from 30hz to 80hz (the assumption is that it's smooth and even there), I would say that Dr. Geddes approach is a much simpler and more economical way to actually achieve those levels than one or two large very high powered subs. The only area where I would concede that a traditional large x-max low sensitivity driver makes more sense is in trying to achieve 115db's below 30hz, where box size will largely not allow a high sensitivity version.