I've never heard the Gradient SW-57, but I suspect it would be more worthwhile than their SW-63 (which I have listened to). My problem with the SW-63 is that it doesn't add much to either the dynamics, or low bass of the ESL-63s - for $3k, I'd want significant improvements in both. They aren't really supposed to be SUB-woofers... they are more intended to extend the speaker dynamics by relieving the 'stats of upper bass. But, the 63s aren't really that weak in those areas to start with, while the '57s really are. I can imagine the added dynamics could be worth it on the '57s... at the very least, adding a Gradient woofer would protect them from being over driven (the '63s don't self destruct), and burning themselves up.
The real problem is price. A good pair of '57s will set you back between $800-$1500 - much more if you want fully refurbished. If you can find a pair, Gradient SW-57s sell for $2k & up. So, a good working, but not fully rebuilt, system is about $3000, more like $5k if you want new panels & a warranty. For that money, you could get nearly new Quad ESL 2805s... maybe even 2905s! If you want improved bass & dynamics in a Quad, That's the way to do it. Even a stacked a pair of '57s would be more cost effective, but then you'll still need to plan on rebuilding them soon... the last '57s left the factory in 1980 after all.
As an answer more to the OP, after trying several, I've had the best luck integrating a Vandersteen sub with my '57s... actually a pair worked really well for both the '57s & '63s. Second best match was a pair of Velodyne ULD-12s - these work better than the larger ULD-15s or 18s with the Quads, they are much less expensive, and have their amps built in, so a bit simpler as well. I don't think I have more than $500 into the pair I have now, and they look/work like new.
As a bargain basement Quad system, a good pair of Quad ESL57s, and one Velodyne ULD-12, plus some DQ-10 stands could be assembled for between $1000-1500. That's a great sounding system, but if it were my money, I'd be looking at ESL-63s on Arcicis.