Good point on diminishing returns and that's a big one, how much more money are willing to spend to get that upgrade in sound? I have switched out my gear throughout the years as some of my friends would say at an excessive rate...some of the changes were in the right direction some were not
For some examples;
From the old school Soundsream reference to the Phoenix Gold zx series we're a step backwards
A wierd one was from the old RF punch HD amps to the DSM's I also didn't like the changes as well
One in the right direction was from JL slash to the Audison Lrx this one was huge!
From the Audison to the Arc I was indeferent, lateral move even though they both sounded very different from one another.
I have tried so many different amps, for that I feel blessed (though the process of buying high and selling low is getting old!)
The sound difference were'nt always a huge difference but there was always suddle changes