@fortepiano50 When I owned Quad63USAs I also found the best matches were tube amps in my case Berning ZH270 and Dehavilland 845 monos. I'd suspect that the Atma-Sphere M60s suggested would have also been a great match.
OTOH, I understand SS adding punch. My Soundlabs are run w/Krell FBP-300 which trounced hybrid Lamm monos that I originally paired with them.
IMHO, the Krell, Plinius, Accuphase & Spectral would all work fine, but I'd seriously consider tubes if you are open to that. One caution w/Spectral, when I bought DMA amp and preamp many years ago, I recall some atypical impedance specs that made use of both their pieces a best practice; some of their gear isn't suited to mate with other brands to quite the same extent as others. Do your research to be sure it's a good fit w/your pre. Cheers,
Spencer