I feel I must put in a plug for Heathkit UA-1s. I've tried several amps with my 57s (which have treble panels rebuilt by Mr. Picquet by the way), including original Quad IIs, Dynaco ST 35s, a Bedini 25/25 and even a Red Wine Audio Signature 30, and the Heathkits are my personal favorites.
I was initially interested in trying the Heathkits when Sheldon Stokes commented on his website that they are his amps of choice for the 57s, and I thought it would be a nice cheap way to get started (as a matter of fact, a pair went for only $350 on eBay earlier today). I personlly find them to be a VERY synergistic match and currently have a pair being updated/modded by Bill Thalman of Music Technology(formerly of Conrad Johnson).
Don