SET amps tend to cope less with difficult loads, and the modern Quads tend to fall into that category. Even the 57s, which were a complex load but don't need (well, handle) that much power, tend to work much better with PP amps IME. You could try one of the monster SET amps, a 211 or 845, but my own experience with an 845 with 57s was not great.
My personal view of SETs is that they are the best way to amplify, but they need the right speakers. Most mainstream speakers are not suitable for SETs - I use horns with mine - indeed only speakers specifically designed for flea powers (Audio Notes, say) work for me. Some here use big SETs like 845s with mainstream speakers, I guess, but my view is that you should choose the speakers you like and then get the right amp for them.
Tim de P reckons that any of his amps will drive Quads, so you could try Bourney's 859, although again I used one and it was pretty easy to get it into clipping...