Sub for Quad ELS-989

Vandersteen 2wq, JL Audio Fathom (F112 I think depending on the room size), Rel B1, B2, or B3, once again depending on the room.

Macdad is on right trail.if you think your speakers are working properly but large scal orchestra or anything below 40Hrz or 30Hrz is setting them off best thing would to be to crossover the signal before it goes to Quads and a sub.You are doing HT so super depth a sound pressure level (SPL) is not the thing you want.it's speed and therefore pitch definition.If you have a pre and amp getting a good crossover like a Bryston 10B Sub is not cheap but works great.Otherwise a Velodyne SVS-1 will do same AND allow for tailoring EQ to room.

Vendersteen is cheap and does not go to low but is a multi driver and faster.REL works in same way fast and speedy.Say a Storm or above (though entry Strata is ok).They have newer models (B series) but think older ones were better.There is a an off shott from REL MJ Acoustics that make similar subs.Some large single drivers will do job like Fathom but at a price and maybe without finesse unless you really pay for it.I onc spoke to Maggie folks and they weren't hot on subs but said don't use a large driver anything above 8" if you do go multi driver as above.Maybe most flexible sub is Martin Logan Depth.Has three 8" drivers and speed for electrostatics like theirs and yours.It's also got enough power to do home theater or work with wide variety of speakers.That and SVS-1 is set up I would go with iof I get back into monitors or ever went planar or electrostatics.Speed and pitch definition not "Boom In The Room" and means to high pass your Qauds is what you want.
Chazz

Check out the Paradigm sub 25 or the new Sub One (a monster that is supposedly "fast" in that it uses 6 8-inch cones powered by almost 2000 watts). Either would be cheaper than the JBL and the room correction feature is awesome (although it doesn't run on a Mac).