I've been very impressed with the vandersteen 2W series subwoofers. Outboard crossover makes for very seamless integration with good quality mains pair. Work best in stereo pairs, of course, and can drive all but the largest rooms to any reasonable level. Tight and precise. Adjustable "Q" on the newer line. Kinda difficult to find good used ones, however (in my experience). Even new, reasonably priced.
