Why wouldn't I use listening to Led Zeppelin or Rush to evaluate an amplifier?
Heaven forfend, I don't want to start a genre war. Let's just say I prefer to use acoustic music for equipment evaluation.
I do too, Tim. There is nothing that is more important to me than believability when it comes to performance with unamplified live music as a reference.
However, rock is great, even essential in my view, to evaluate bass and rhythm & timing. Problems with the latter are less obvious on classical and classical avantgarde, my musical main diet, but on a subliminal level they will also be important there, and rock reveals it all (it doesn't need to be "classic rock"). Jazz is also good for that, but I found that especially digital components tend to have, or used to have (it was a really bad problem in the past), rhythm & timing problems with in particular rock. Fortunately, my current digital is a veritable rhythmic animal, and can compete in that area with any turntable.
So yes, I think Caesar was entirely correct to use listening to Led Zeppelin or Rush to evaluate an amplifier, at least particular aspects thereof.