The Quads to which I was referring are the originals ("57’s"). Modjeski says he used that speaker for evaluating his design choices in the development of his RM-10 amp (35w/ch in it’s Class A/B version, 25w in the Class A version). The Quad 63 and it’s descendants are a very different story.
Modjeski offers not only push/pull Class A/B amps, but also low-powered single-ended amps, and an OTL (as well as an ESL speaker with a dedicated direct-drive---no output transformer---tube power amp). The Quad 57/OTL amp is a classic combination (especially the Futterman OTL’s), but Roger makes the case that an OTL amp is in fact a poor choice for use with that Quad. His reasoning was explained in the now-dormant Audio Circle Music Reference Forum.