What is your experience with the law of diminishing returns ?

Yeah, we can try to fool ourselves and at times we can even be successful.

Cables and power cords are components and as such they can sometimes give as much sound change as active components.
However, speakers might not be good enough, so whatever you replace in the chain give about the same magnitude of change.
Or, your system is a total mess, completely in disarray, and whatever you do it won't matter much - same mess.

Basic music lover listening.. Used pair of old J B Lansing speakers, found at Salvation Army Store $45. From another type of same store Marantz 2275 for $30. (BACK IN 1993) A NAD CD player from Goodwill $8. Turntable sitting in closet for 20 years but also from Goodwill $4 new cartridge $70. Free IC (from me). Friends system.Plays music I could listen to all day.    

Total cost (plus a lot of luck) $157. Quality of playback. 90%   OK maybe 85% of the best possible.     
If you spent $20,000 get you up to 92%
Spending another $75,000 gets you to 97%        
Spend an additional $500,000 and you are up to 98.7%THAT is the power of diminishing returns.

I think that Elizabeth has it right in terms of marginal cost / marginal gain curve.   One might quibble about price point (not a lot).

Then the owner’s financial condition as well as sensitivity to the curve must be factored in.

For myself, I lust after the new 30.7 Maggie’s, but a house downsizing leaves me a room 23 x14, not quite large enough. At the end of the day, I love where I am I terms of my system, but we always see the greener grass smiling at us, LOL.

Also, often lost are the relatively inexpensive tweaks, I.e. room treatments, etc.

Reminds me of the old line, "What's the difference between a $2.00 cigar and a $10.00 cigar?"  Answer:  "$8.00"