Best Subwoofer for Quad 988'S

You may want to read Paul Seydor's review in TAS under his subwoofer in which he thought that the REL matched well with the quad.

Kjl, try to hunt for a pair of Grandient subs (plus x-over) on the used market. They blended ideally with the old Quad63s, which you could place on top of the subs. That would be my first choice.
My second choice would be one, or a pair Carver Signature subs, which I would run independently directly from my preamp. I'd set them at between 50-80 hz. They have to be blended in very carefully, because they are slower than the Quad. It can be done, with proper room placement, but it takes time and a lot of experimenting.
A better solution, but with a bad wife acceptance factor,would be a pair of old Maggie bass panels, the speed of which almost equals the Quads. You'd need a good active X-over to get the best results and cut them off at about 100hz. Again I would run them independently from your main system, i.e. let the Quads go all the way and just use the XO to cut off the higher frequencies of the Maggies.