You should also check out the "MTX PS15" sub. I own 2 of them along with a "VMPS" Larger sub. I bought the MTX's after reading a great review of them during an exhaustive subwoofer survey in "Widescreen Review" (which you can find on their web site). They list for $599, but sell in the mid $400's. These are really good performers and sound terrific on both music and movies. And they also have remote control!
If you have the room and a good spare power amp, I'd also look at the "VMPS" Larger sub. The price is VERY reasonable, but this is an unpowerered speaker, so you do need to add a power amp and crossover. This is also a very large speaker, but you can not believe the bass performance this thing is capable of. It is actually down right scary at times, I mean, my house seems to move!
VMPS does not try to cheat the laws of physics, they use several big drivers in a big solid enclousre. There is NOTHING wimpy about it.
Now that's a serious subwoofer.