Should I buy a Class A Amp.

Could it be said that Class A amps, like Pass for example, play better on certain types of music? Better classical than rock, for instance?

Actually, it was I who asked this question, and I had not ever heard this said before, myth or otherwise.

Fact is, with my Pass XA30.5, that is exactly the impression I get. Better on some types of music than others. Relatively speaking.

Ideally there should be no difference for an amp to play any kind of music equally well. In reality, there is. You don't want Kondo to play jazz/rock fusion. Some amps manage to strike a good compromise. But it's not only amps, it's also speakers etc. If I could I would have two completely different set-ups - one tube, the other either solid state or hybrid or also tube but different.
Atmasphere's mythology is that there are no better amps than his and no better ideas than his. I'll leave him to that.

What preamp do you have?

Atmasphere I've never heard one of your amps (I live in New Zealand) but from what I've read they would indeed do just as well on rock as on classical or jazz. Same with the big Pass amps. They're very expensive though, and when you get down to the slightly compromised amps there are trade-offs that mean some are better at rock than others. Bass and PRAT are essential for rock and some amps just don't deliver because they have other priorities.