Been on a similar quest, but want a "real" subwoofer (in-room F3 = 20 Hz). Cheapest/smallest I could find was SVS SB-1000 ($499, 13.5" x 13" x 14", F3 = 24Hz) and Rythmik L12 ($539, 14" x 14" x 15.5", F3 = 18 Hz). Either should be good quality pieces, especially the Rythmik.
Note they would hook up to speakers with F3 = 42 Hz, so why bother with a "sub" that is only rated down to 35 Hz?