It was Gene.
I believe I have also heard qualitative differences between amplifiers, but Gene's comments were indicative of a greater magnitude of difference than I've ever heard, and I've owned Levinsons and class A Krells. The differences I hear manifest themselves in listening fatigue, rather than any ability to pick them out in A/B testing.
I have regular access to a friend's system with Pass Labs amps, though I've never A/B'd them with anything. He also runs different speakers than I do, so any comparison is utterly meaningless. I can say that they are awesome amps, exquisitely built by passionate people in California, and that they are visual works of art as much as audio devices. Nonetheless, if you are worried about price performance compared to more mundane products, or you're wondering if there really are differences between an XA160.5 (which my friend uses) and a Crown XLS, you're not in their target market.
In the Stereophile review of the XA30.5, JA measured some very unusual distortion characteristics, in that the distortion spectrum was nearly all third harmonic. That has always fascinated me. I have wondered if such a different distortion spectrum would be audible.
In the past couple of years I've lost my passion for spending many thousands of dollars pursuing very subtle differences. Well, at least with amplifiers. Part of it could be my objective skepticism that any differences that are at least 80db below the fundamentals could result in overt audible differences.
That stuff aside, if you have to have a dream amp, you could do a lot worse than Pass Labs stuff. They really are awesome products built by awesome people. But anyone worried about the price is missing the point.
