After 20 years of Quad experimentation, the Esoteric Audio Research 509 gets my vote. Effortless and neutral when much higher powered amps suffered. The capacitive load probably won't work with any Class D amps because of the feedback scheme. Both aforementioned amps have tertiary feedback windings in the outputs. Those windings prevent the feedback from getting out of phase due to the capacitive nature of ESLs -if I understand it. Otherwise, the best Class A sand amp you care to buy. I have an old Bedini 100/100 workhorse from the eighties that I keep around just because of this. It is not as transparent as some of the modern amps, but handles the load with aplomb and works in a pinch. BTW, the Bedini 25/25 was hot with the 57s back then...
