If you can find them used the Velodyne CHT subwoofers match the CHT speakers well. Plus the grills will match. Even though I'm replacing it with DIY subs, I've been happy enough with my CHT-10 and CHT front rows.
AV123's x-sub would also fill this pretty well and be in your budget. I and some others have had trouble getting it to play loud enough when connected to the high level inputs though. The line level works more then fine.
As far as subwoofer placement, I think that will be important with all subs. Placement has to do with playing with room acoustics and no subwoofer will change the physics of the room.