The Finnish company Gradient made OB/Dipole subwoofers for both the QUAD 57’s and 63’s. Though the subs used dual dynamic cone woofers, they being employed in OB/Dipole fashion allowed them to blend better with the QUAD’s (because of the inherent advantages of OB’s, and the dipole characteristics shared between speaker and sub) than any sealed, ported, or infinite baffle sub.
Ad a reminder (I have spoken of them here before), Rythmik Audio and GR Research designers Brian Ding and Danny Richie have collaborated on a joint product, their OB/Dipole Servo-Feedback Subwoofer. A very unique sub (or even woofer---it can be used up to 300Hz), THE sub for QUAD’s, and any other dipole speaker, whether ESL, magnetic-planar, or ribbon. Or even dynamic---Siegfried Linkwitz used OB/Dipole subs in his loudspeaker kits, very similar to the Rythmik/GR (and almost as good ;-) . Whart, you should really look into the Rythmik/GR for use with your QUADS, and even your horns!