Diminishing returns can’t possibly apply in this context, anyway.
Sound quality is by definition qualitative not quantitative. It is meaningless to say that one product sounds twice as good as another or for that matter any numerical factor of goodness, whatever that is, better.
The Wilson Benesche costs ten times more than say a Klimax LP12 or whatever. But no one can authoritatively say it is or isn’t ten times better sounding.