Best Subwoofer for Quad 2805 under $5k

The Finnish company Gradient made a dipole sub for the Quad 63 in the late 70's, I believe it was. it was a pair of 12" woofers in an open baffle H-Frame, the woofers facing in opposite directions and wired in opposite polarity. At least theoretically this is a design well suited to use with a dipole speaker.

There is a sub design currently available just like the Gradient, but using servo-feedback on the woofers. The plate amp with servo circuitry is made by a company in Austin, Texas named Rythmik (Martykl mentioned his sealed Rythmiks above), and the 12" woofers are a collaboration between Rythmik's Brian Ding and well know speaker designer Danny Richie of GR Research. The sub is offered as a DIY kit only, the user needing to make (or have made) the H-frames into which the two (or three) woofers are installed. Anyone wanting dipole subs to mate with dipole speakers should definitely check out the GR Research website for more info.

The JL subs will work well, but you might want to consider the latest BalancedForce subs from Martin Logan. They are designed to integrate well with their electrostatic speakers and retail at $2,995 (210) or $3,995 (212). With their optional Perfect Bass Kit (PBK) you can optimize your setup using the provided mike and software with your PC.

for music, rel g1

The problem is not the subs but the room. Hence use multiple small subs and dsp room eq like the DSPeaker 8033.

Having used a variety of subwoofers (many named above) with Quads ranging from the 57's to the ESL-63 to my current 2805's, nothing comes close to the Celestion 6000 subs. They are true dipoles and extremely fast. They go low enough for me, especially with the 2805's. I have always positioned them next to my Quads (on the inside/side) rather than putting my Quads on top which really doesn't work well with the larger Quads. Not sure how available they are and they're really heavy and so shipping costs would be high, although worth it!