The British company Gradient made an H-Frame Dipole Open Baffle subwoofer/stand for the Quad 63's in the 70's or 80's, that did an okay job using two woofers and a pretty low-powered amp. Because the design is Open Baffle Dipole---same as the Quads---it blends with them better than a sealed or ported sub. But The GR Research/Rythmik Servo 12" Open Baffle subwoofer system is far, far better. Danny has plans for a W-Frame OB cabinet on the details page for the 12" OB driver on the GR website. If you don't want to build it yourself (the woofers and plate amp come as a complete kit, but the W-Frame is not commercially available), Rueben here on this Forum will build it for you, or a cabinet maker in your area can make a pair from the plans. The plate amp let's you choose the cross-over frequency---80 to 100Hz is good for Quads. It's THE way to subwoofer Quads (I have them with my 57's) or any other Planar speaker. There is a guy here who has them with Soundlab Electrostatics, and I know of one guy who uses them with Magneplanars. Check it out!
