Best subwoofer for Quad 988?

If your friend is happy with the Quad's volume as it is, then any REL would be perfect. Otherwise spend a little more for a pair of Velodyne DD-10s or 12s.

Tom,
I would recommend the Martin Logan Depth which is made to work in conjunction with ES speakers like your Quads. Very fast and musical sub that has three eight inch woofers. I have also used and liked B & W ASW 750 and ASW 675 which were good with two channel music.
Regards,
JWALKER

The best subwoofer for the quad 988 is TBI. Read this review. http://www.videohifi.com/16_TBI_ENG.htm and others from the site.