I like the design of the REL subwoofers. Rythmik would also be another option.I bought a Rythmik F12G from an A'gon member a few years ago. He told me something about setting it up that was spot on and relates to the earlier reference to time and sound waves. He told me to add a few feet when it came to any room EQ setup where the distance between the speakers/sub and listening position was adjustable. My room EQ automatically adds about 1.75' extra feet to the distance. I played around with it and eventually landed on adding 8-9' to the distance between the sub and the setup mic. Actually it's something like 8.3 - 8.7 feet depending on how I have the room arranged furniture-wise.
I have been been able to integrate - for lack of a better word - my single Rythmik sub between my MMGs through this added distance and it truly incorporates itself quite well. The integration is MUCH tighter after the additional distance, the lower frequencies do a much better job of disappearing and melding into the music, and it makes for a fine two-channel listening experience (to me). The sub is spec'd to extend to 14 Hz and is highly adjustable. I set it at a 20 Hz extension (with high damping) for the Bach pipe organ music I occasionally treat my cross-water neighbors to, and it does a fine job of rattling the wall hangings (it that happens to be your bag).
I don't happen to have the room for a swarm in my listening room (that happens to double as my living room) nor do I particularly want one. I'm quite content with what I ended up with after much research, adjusting and listening. But you CAN achieve perfectly enjoyable results that sound very, very good with a single sub when faced with certain limitations.