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.
Pro Musica in Chicago was using Sunfire subs with the 988s last year. No doubt an excellent combination.
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.
Larry Greenhill of Stereophile uses a Velodyne DD-18 with his quads and says the integration is seamless. I imagine a DD-15 or 12 would work in a smaller room just as well.
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.
DD-15 Will work with any speaker.
I have no idea if they're still available, but something called a Gradient was designed to and did indeed match the sound of the Quad 63s. They may work well with 988s or have a version for the 988.
I used to sell the Velodyne HGS subs.While they were great for HT they just weren't fast enough and pitch accurate for Maggie and ESL 63's IMHO.I would try REL imported by Sumiko.Like some other designs (like the $1250 budget champ Vanderteen they use multiple small drivers and therfore are quick.I think they will give you the pitch accuaracy that you want.A big but fast sub that isn't startopherically priced is the Revel at $3K but still say at this or a lesser (or greater) price point think the REL will doa btter job.Left the shop 3 years ago and am not sure about the DD series of Velodyne but from what I read set up is a snap and might be worth a look.BTW I was thinking of giving the 988's a spin myself/Always thought that nothing in mids could beat the 63's but soudstage was too short.placement a bitch,and and you just couldn't egt sub to work well.I know of folks who overt say a 20 years audiophile carrer have gone back to them two or three times but because of the sub intergration problem have given them up.But newer ones are better in all respects.I read in 'Phile that the 989's have less of a need for a ub but the 988's are more of "a piece" so that's what I thought of to replcae my Odeon Tosca semi horns.But think first i am going to give the Infinity MTS POreludes a go round but I will goin you in the sub hunt myself if I go with the 988's or my third choice B&W 803N's.Even some speakers like the 803's where many could live without a sub freak when they hear a good one used as it should be as a SUB-woofer".
Best of luck
You want killer bass with the speed to match the Quads? Here's my suggestion, another diy project. Order a pair of Walsh 200 MK-2 speaker drivers from the folks at OHM Walsh. This will run you about $1100.00. Next, build a cabinet of 2.5 - 3 cu.ft. interior volume, ported, tuned to 18hz and there ya go. Order a Marchand 2-way stereo crossover, an amp known for great bass control and you're on your way.
The Gradient worked very well with '63s; see http://www.restek.co.uk/sw63.htm and search Google for Gradient for lots of comments about them.
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.
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.
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.
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.