No doubt claims for differences are often grossly overblown.@MattHooper , sorry didn't mean to quote you. Not sure what happened.
That is true, but I'd still bet that even in cases where there is a true difference that would hold up in ABX, the majority of the perceived difference is still due to imagination. I think we all underestimate just how much our brain alters the sound we "hear".
I have no doubt that certain amps do sound slightly different, but I'm very skeptical of the "night and day" or even "not subtle" claims I hear. Looking at the 1-2dB audible band deviations of speakers of the same model, I'm quite sure the differences between good/bad amps are smaller than the difference between one's right and left speaker. You don't see people spending thousands of dollars repeatedly buying the same speaker over in over in an attempt to get a better sample, despite it making a bigger difference. Why is that? I think it's because those people know that while sample variation does make a difference, they don't expect it to make a "non subtle" difference. They do expect that with a new amp, though, and so it becomes true. I really think imagination is most of it.
The standard subjectivist audiophile maxim is, "Trust your ears". But I for one don't trust my ears. So for example years ago a compared interconnect cables. I thought I heard differences but I conceded even than the those differences were very slight and might well have been entirely imaginary.
So on the other hand when I obtained my current Purifi amp, I was surprised -- and surprised myself -- by how much more resolved and dynamic the Purifi was versus anything I'd heard before, (even including a Hypex model). I'm reasonably convince the differences aren't just imaginary.