However I would not trust someone that says I don’t hear a difference .
How about from someone who actually designs amps?
Here are a couple of snippets from an interview done with Quad's Peter Walker many years ago:
"Question:
Have you any opinions you'd share on the relative merits of distortion tests,
such as harmonic, two-tone IM, transient IM, or slew rate limiting, as clues to
amplifier quality?
"An amplifier should, within its limits of voltage and rate of change of voltage,
(which is slew rate limiting) if you keep within those two it should be very much
better than any program material. These are the things that are measured at .01
per cent or .05 per cent. But what is listened to is usually a program with 2 or 3
per cent distortion in the first place. That's the least you can get on records,
tapes, and such things. Listening tests are usually not done in this region of .01
percent distortion. I'm quite convinced within that range the amplifier is just as
perfect as you like to make it. It's quite possible to put 50 amplifiers in cascade,
each one into a load, potted down into the next one, and to listen to the 50th one
or to listen to the first one, and the sound will be virtually the same.
...
The peripheral effects are what get people into trouble. You can
see why you find these differences in amplifiers. You can always find them. If
people test two amplifiers and say, "These sound different," there's no magic in
it. Spend two days, maybe a whole week in the lab, and you find out exactly why
they're different and you can write the whole thing down in purely practical,
physical terms. This is why these two sound different, and the cause is usually
peripheral effects. It is not really a case of good or bad amplifiers, it's that the
termination impedances are wrong, or something of that sort. "
"TAA: How do you rate the merits of listening tests to instrument tests?
PW: We designed our valve (tube) amplifier, manufactured it, and put it on the market. and never actually listened to it. In fact, the same applies to the 303 and tile 405. People say, "Well that's disgusting, you ought to leave listened to it." However, we do a certain amount of listening tests, but they arc for specific things. We listen to the differential distortion - does a certain thing matte' You've got to have a listening test to sort out whether it matters. You've got to do tests to sort out whether rumble is likely to overload pickup inputs, or whether the high frequency stuff coming out of the pickup due to record scratch is going to disturb the control unit
But we aren't sitting down listening to Beethoven's Fifth and saying "l hat amplifier sounds better. let's change a resistor or two. Oh yes, that's now better still." We never sit down and listen to a music record through an amplifier in the design stage. We listen to funny noises, funny distortions, and see whether these things are going to matter. to get a subjective assessment. But we don't actually listen to program material at all."