There are diminishing returns and there are values. When I bought my first external DAC years ago it was a Denafrips Ares II. Made a huge change to my system. I caught the bug. Year and a half later I purchased the Pontus II at 2x the money. Was the Pontus II twice as good as the Ares II; I would say not but was it worth it; I say definitely yes. The improved sound was not linear to the increased cost but the improved sound had definite value to my listening experience.
I could make comparative equations across most electronics in my humble opinion. How one would do this with loudspeakers is beyond me. A $5k system can be really really good. Would a $10k be twice as good or $20k be four times. Probably not, yet does the increase expenditure increase the listening pleasure; does it add value to your experience?

