In all my personal experience in this great hobby of ours, IMHO, there is no other item in high end audio that fall under the "Rule of Diminishing Returns" like loudspeakers.
I have to disagree.
I don’t think that diminishing returns applies anywhere in audio.
For an example; my rich cousin was recently deciding between Pass Labs, or CH Precision at a substantial difference in price (to drive his Von Schweikert Ultra 9 speakers). What did he get for the 5 figures he had to lay out for the difference? A small, but noticeable improvement in soundstage size and depth, a bit better layering. Maybe a bit better attack/decay.
He spent more in just the difference between the amps, than most people spend on their entire systems. Ask him if he passed some point of diminishing returns, and he will tell you, emphatically no.
Remember, there are 100’s of millions of people out there, who listen to all their music as MP3’s, on their smart phones, on $20 earbuds, that think that even people who have spent a modest few grand on an audio system, have passed the point of diminishing returns.
Seems to me, that the point of diminishing returns for a lot of people, mysteriously starts at the point that their ’sour grapes’ kicks in.