Best active subwoofer for 4 Quad ESL 57

i suggest you look for a muse model 18 (they're no longer made), together with a "personality card" specifically designed for the quads. for the latter you might want to contact kevin halverson at museelectronics.com. i had a couple of muse 18's that i mated to several different "main" speakers. they are the fastest subs i've ever heard, which is a character, of course, demanded by quads. the downside is the 18 is BIG, a cube that's roughly 36x36x36 inches.

I am currently using a REL "Storm" with my ESL 57 and while it does not play the lowest notes, it certainly augments the whole lower fundamentals.

Integration is seamless & and is able to keep up with the panels.

I use a pair of JANIS W-1 with a pair of Bryston 7s and a Bryston xover set at 24db/octave balanced. Need a big amp to control/integrate the woofer even though the speakers are efficient.

The new spendor is the first sub I've tried that works.I've been using quads for 20 years.

When David Hall designed the first Velodyne sub he used ESL 57s as his speakers. sonically he was trying for a fast accurate and low distortion sub. I think that you should not overlook the obvious...i.e. a Velodyne.