I use the Infinity BU2 12" downthrow sub with 100 watt amp built in, It runs off my Harmon/Kardon
AVR80 MKII THX receiver in a 13' x 13' room and I've been quite satisfied with its perfomance for over a year now. Infinity has stepped this model up to the BU150 with digital circuitry. You can still find the BU2 for around $300 or the BU150 for somewhere around $450.