On amplifiers, he does say that amplifiers of a similar design, matched in levels and run below clipping are indistinguishable from each other in listening tests. He never said that all amps - tube and solid state, set and push-pull, all sound the same.
That's sort of true, but Aczel did assert that if tube and SS amps measure alike they will sound alike. I believe that too.
With amps I wonder if things are a little more complicated than Aczel asserts, but, to be honest, I'd rather be on his side of the argument than one asserting differences between amplifiers. A lot of good amps these days don't have a single measurable artifact of noise or distortion above 80db below 1W of output. How any differences could be audible even in the quietest room on an awesome speaker system is difficult to imagine. Other than more power for higher clipping limits, or more stable performance into very complex loads, you begin to wonder about what could possibly improve?