It's probably not something you'll be able to discuss in terms of math. If you look at the numbers, you're right. Push-pull offers lower distortion. Well-executed solid state even lower distortion. But to a SET purist, push-pull is "flat and uninvolving" and there is no such thing as well executed solid state. They're lured by the warmth of SET magic.There are two things that may explain some of the reasons why people prefer SET. For one thing, Class A amplifiers have no crossover distortion. The entire audio signal is handled by one amplifying element. And for another thing, the distortion that is made is largely low-harmonics, mostly second and third. This is mostly due to the fact that the tube conductivity isn't perfectly linear, and doesn't have anything to do with more complex things like feedback nonlinearity. A tube clips gracefully too, not with sharp harsh-sounding square edges but almost sounding like it just won't go any further. So without resorting to romantic adjectives, you can see some things that may be part of the SET magic.