they are not very sensitive but they can also not tolerate very much power - they are also essentially big ol' capacitors... so, yeah, they're a little tricky to match with amplifiers ;-)
The Quad II amplifier was about 15 watts; generally I think that folks 'in the know' will prefer to use 15 to 25 wpc amplifiers, with vacuum tube amplifiers being preferred (mostly due to the speakers' load characteristics). A McIntosh MC-225 is a very good choice. Quad's SS amplifiers of the 1970s worked well with them - and the Quad II was a fine amplifier :-)
I usually use PP EL84 with mine, but they are quite acceptable with SE2A3. I didn't try any hard, hard rock 'n' roll or big-scale music with 'em, though (which probably goes without saying). If I ever manage to find room to set 'em up permanently, I mat use a PP 7591 amplifier with them... one of these days I am going to try 'em with my Sherwood (whichever model it is... S-5500II, maybe?!).
Pairing ESL-57s (especially earlier models) with high power is a recipe for arcing = a recipe for disaster.