One manufacturer told me they are working on a "Statement" sub that has six 12" drivers and a 4,000 watt amplifier. I know this sounds ridiculous at first, but the real idea is that, at normal listening volume, you will get 10 Hz at very, very low distortion. Some movie soundtracks have this kind of low bass. And for transient peaks, again, you will get everything you could possibly want.
I don't think it is going to be very tall. Perhaps 30". The enclosure is a trapezoid, with two woofers each on three sides, and the amplifier controls on the rear.
Two questions: 1) does it go to "11"?; 2) do I get to review it? :) But seriously, with two woofers firing on three sides, I can only imagine the issues with standing waves and nulls.
On a similar note, I've been intrigued by the redesigned version of the Elemental Designs A7-900: http://blog.edesignaudio.com/?p=653. They have a video showing the building of the sub that is quite impressive to watch as well: http://blog.edesignaudio.com/?p=513. It's just too bad that our condo board would certainly take action against me if I tried to add something like that to my system right now.
If the cost was reasonable, I might consider this. But I'd rather have a pair of them with three 12" drivers and 2000W each, to smooth out the response. Right now I'm using four 15" drivers and 2500W peak. Specifically to handle the extremely low frequencies with minimal distortion and maximal SPL.