Ok, not sure what absolutist bible exists that portends the efficacy of all amps for all music types! THAT is lunacy. Classical, jazz and rock can favor very different frequencies and musical tastes.This may come as a bit of a surprise, but designers don't 'voice' an amplifier to work with a particular genre of music. There literally is no way to do that!! What does happen is some amps are quite literally better than others- lower distortion, wider bandwidth, that sort of thing, and you can hear that.
Those amps that are better will sound better on all types of music. If it happens that someone auditioning two amps where one is simply better, they may come away with the impression that one favors a certain kind of music. But that is simply because they didn't listen to all genres during their audition (which really isn't possible- there being so many) and one must keep in mind that all recordings are flawed- rock , jazz, classical and so on.
If an amp is limited in bandwidth, it will suffer for rock as it does classical, jazz or folk. If the amp makes unpleasant distortions (such as higher ordered harmonics or IMD) it will sound harsh on **all forms** of music.
No lunacy- just science. This really is a persistent myth. The same applies to speakers- for example the JBL L-100 (recently re-issued) was long said to favor rock, but it does just as well on classical, country and jazz. Again, quite literally, there is no way to build a speaker so that it will favor a certain genre. You can boost the highs and lows and maybe that's more pleasing to you for rock, but if you play a different genre you will find that the boosting of the highs and lows serves just as well there- IOW, such colorations are likely working to compensate for colorations elsewhere in the system where this sort of thing is perceived.