My recommendation is the ACI Force subwoofer. An outstanding little sub, with compact dimensions, that is flat to 20Hz, +-3db. Its performance is identical to the giant-killing Titan II LE (also from ACI) except for slightly lower output. Pricing starts at $749 for basic black to around $850 for real wood veneers. Check out specs at http://www.audioc.com.
What to look for in a sub?
I've been pretty happy with my HT setup using a Yamaha RX-V1, a mix set of speakers from Definitive, Paradigm, Acoustic Research, and a really old Fosgate subwoofer. (Most of the fund goes to the 2-channel audio setup) I am moving to a new house with a smaller living room, and the sub is too big. (30"W x 30"L x 15" H) I need to find a replacement sub that will fit into the new living room. What criteria should I look for in a sub? Also any suggestions for a good sub (new or used) in the $800 range would be greatly appreciated, active or passive. (I currently have a Marantz 50w monoblock driving the Fosgate).