Best subwoofer for Quad 988?

If your going to go a DIY or get a passive job get a used NAD 218THX amp.225 wpc stereo and if you read the stereophile review the reviewer saidf it might be a bit thick in the mids but had more bass slam than any amp hje had revieed.You can pick it up used for less than $500 and if I remember corerctly it had like 700 wpc mono.Another budget candidate would be the 300 wpc stereo Adcom 5802.Gobs of power for less than $800 used.But in all myu experience nothing had the bass "traction" and controll of Krell.One of their class A/B amps from 150 or 225 on up converted to mono would givbe you a "grip,slam,and control" that others just can't deliver.For full range even their class A amps have a brittle/metallic mid range that some either like or overlook because of their virtues fo clear treble and bass control but for a passive amp or DDIY it would be the best.Freinds I have in pro audio might say Crown (bullet proof) like a DC300 but you can go with a lot of options with a bass amp you might not consioder for your mains.
Chazzbo

Don't overlook Hsu Research. Dr. Hsu designed the originals to go with his Quads.

I get excellent results with a pair of Rega vulcans(the drivers are small so this means speed).They are based on the REL subwoofers set up(speakon connectors),no roll off,the main speaker and the subs work together.This may sound strange,but I believe in using british subs with british speakers.Both the Rega and the Rel are UK designed.Look them up.They are fast and you will not know you have subs in your listening room,just the bass in the music.I have used various Quads since 1985.I can talk source and electronics,but,thats on a different day.

TBI! Read the Stereo Times review by Dan Dzuban. He owns a pair of Quad 989s. http://stereotimes.com/speak112204.shtm

I'm running a TBI Magellan VI with a pair of 4 way full range electrostatics and the transistion is seamless.

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.