Quad 57s excel at low volume and have few peers in this area, in my opinion. That said, they have limitations many cannot live with in the long term. The Quad 988s are quite different from the 57s, with many differing opinions on them. For integrating with the Rel subwoofer, the Vandersteens may be the better choice, but only you can decide which is best overall. You didn't say what amplification you have, which should enter into the considerations unless you are thinking of changing that, too. And what about your room?
quad 57 vs vandersteen for low volume listening
I listen at low levels to prevent hearing loss and exacerbating tinnitus. I had decided on Vandersteen quatros (not sure about sock vs wood), but a dealer suggested I consider rebuilt Quad 57's. The Quad cons of not playing loud and less treble emphasis are pros to someone with tinnitus. I clearly preferred the coherent, real sounding Vandies vs Quad 988's side by side. (I will audition the 57's soon, but can't do a side by side with Vandies) I am currently using a Rel Strata III with Vienna Acoustics Mozarts. Seems the Rel wouldn't integrate tonally with an electrostat. I'm not sure I can live with the missing low frequency harmonics in the Quads that I think are fundamental to creating the illusion of being in the room with with musicians. Let's hear from all you 1st order phase aligned and electrostat proponents!