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.