Best subwoofer for Quad 988?

Do a DIY project. I would suggest (4) 10" subs in each of two cabinets. The (4) drivers will be orientated from top to bottom in small sealed subenclosures. The cabinets will be made of 2" MDF on all sides.You can power each driver with one half of a Parasound 2200 mkII's.You will need two per side.A bryston crossover will do nicely.This system might set you back 5-6k but it will be your creation and no one else.Good luck.

Another DIY project would be a pair of 15" Adire Tumults on a largish dipole baffle.You could run Bryston 7b's on each with a TL circuit to tailor the bottom end.That would be cool.3k or so.

If you do plan on a design other than your own, a couple of Vandersteen 2W will do fine.I used a pair with a pair of Spica TC-50's and the things just sang.Good luck.