@asctim And what I said about changing just one thing at a time in audio being tricky is indeed illustrated by your experience.
The two amps are quite different in many of their characteristics. I know Topping are very committed to being "transparent" - adding nothing to the source material. Like it sounded in the studio. This is actually something that most people probably haven’t experienced first hand.
Other amps (especially tube amps) add 2nd and 3rd harmonics, and I would say that the Hafler amp probably does too from your description. There is much more involved, but that is sufficient for now. There has been research into why the brain interprets these aspects as pleasing.
So, a bit like comparing a photo of something (perhaps one by Ansel Adams?) with a painting of the same subject.
What is important about all this in the context of the thread is that this comparison cannot be captured at all well by the concept of marginal return, because the technologies are rather different.
What must be compared are at least two recent releases by the same manufacturer, just at different price points. One may cost twice as much as the other one, and the sonic difference rather slight. If any. One that only a reviewer may be able to discern.
But I dunno, the more I think about it the more I think that anything like that (demonstrating declining marginal return) is just giving some warped credibility to whatever it is that Mr Harley was on about, and I no longer want to do that - he made a claim so its his responsibility to prove it.
This is a dismissal of the nature that is known as Hitchen’s Razor.