Gotta step in here and throw in my support for the Martin Logan Depth(i). This little sub delivers the goods. It has 3 - 8" drivers, can push out 900 watts peak, and sounds fantastic with music as well as movies. If transient quickness and in-the-guts impact are both important to you, be sure to check this powerhouse out. Oh yeah...and it's good for an honest-to-God 20 Hz; mine goes to 16Hz (-6 db) in my room. That's bone-crushing, rattle the silverware drawer bass, fellas.....
It ain't the smallest or cheapest sub out there, at ~2K. But they can be found on the 'Gon for half that just about anyday of the week.
~VDR