@prof, knowing how well the OB/dipole Gradient sub integrated with the QUAD 63, you may be interested to know that there is now a contemporary OB/dipole that does everything the Gradient did, and more. It was developed as a team effort between Danny Richie of GR Research and Brian Ding of Rythmik Audio, and it is really special. It is comprised of a pair of 12" servo-feedback controlled (Rythmik’s reason d’etre) woofers installed in an OB H-frame. It is THE sub for dipole loudspeakers.
Danny Richie showed at RMAF for a few years, using a pair of the OB/Dipole subs at the front of the room and a pair of sealed subs (Rythmik F12G’s) at the rear. His room was voted "Best Bass At The Show" three years running.