@Sprite - Looking at REL and Sennheiser subs, while I'm sure either will fill in the low end very well, I always have a problem with subwoofer specs either just listing their frequency ranges (like Sennheiser's "
27 to 80 Hz") or listing the range with the dB (like REL's
24Hz at -6dB). REL's spec is better as you don't know what Sennheiser is using to claim that 27Hz as it might not even be audible and the "real" low end is actually more like 35Hz. The problem with the -6dB spec is the sound output that you can actually hear really drops off before it reaches, in this case, the 24Hz they claim. Not including the dB at all or using -6 for the dB, you're really just trying to pad your results to make them look better than they really are.
Personally, I'd go with something like the
SVS SB-1000 Pro which uses -3dB as the cutoff for the usable low end and also goes lower at 20Hz.
Keep in mind placement in the room matters, too, as null waves can make or break the sound bouncing off the back wall and cancelling out the sound. That placement will be the same regardless of which sub you choose.
One other curve ball - have you considered transducers? They mount to your chair/couch and vibrate with the LFE channel giving you the rumble effect of a large sub without the volume.