For sure, there are measurable differences between the
same amplifiers. All the individual components that they're built with will measure different. 5 nines % of the time you won't be able to tell the difference audibly.
I've never disagreed that amplifiers will sound markedly different. At least when comparing apples to an apples. I am a believer there are subtle differences that can be discriminated audibly, however. Then is this truly better or just preference? The human ear isn't a measurement! I like my tele, my friend likes his strat. I am convinced, and it may take a lot of listening to a lot of different material, that my B&K and Acurus are subtly different. I have no emotional attachment to either of these.
And, the amount of "improvement" per dollar goes exponential. I used to race bikes, I won more races on a relatively inexpensive bike than I ever did on the super fancy ones. But I sure like my current bike better than any other I've ever owned