I currently use a pair of Quad ESL-63's that I have owned for around 5 years. I have owned several different amps. The amps I am currently using are by far my favorite. They are a pair of Cary 805c's. They have more than enough power to drive the Quads, but tend to be slightly bass shy (of course, Quads are a little bass shy by nature). If you don't want to spend the $$ for the 805c's, try to find a used pair of Cary SLM-100's. My previous amps were a pair of those with KT-88 tubes. You get about 95% of the 805c sound and a little extra bass as compared to 805c's.
One last comment, the first amp I used with the Quads was an Audio Research integrated, and I have also heard Quad 988's powered by a pair of VTM-200's. To my ears Quad / Audio Research does not make a good match.
One last comment, the first amp I used with the Quads was an Audio Research integrated, and I have also heard Quad 988's powered by a pair of VTM-200's. To my ears Quad / Audio Research does not make a good match.