What is your experience with the law of diminishing returns ?

No experience so far. $10k power cord replacing $1k power cord with $5k preamp or power amp, anyone ?

The law cannot be applied if you got the wrong component, including cable, regardless of price.
The system doesn't have to be high end for you to hear even small differences, but it has to be good enough and very sensitive.
No experience or "brain teaching" is needed.

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.

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

This is unusual, I think, same brand speakers for half the price being preferable overall. Were the electronics source and cables the same when you were auditioning ? 

I will give an example within my current system. Power cords, yes again. One is Purist Dominus used to have $2100 retail price, another Top Gun by CPCC used to cost $800.
If I compare them on the integrated amp the sound difference is big enough to justify the price difference, but if I compare them on either phono stage or cd player, it is not. For me. Dominus is still better in every respect everywhere. Sound difference on regenerator also would justify the price difference.
Since I bought them both used, for $120 and $650, I would put Dominus everywhere if I could find more of them for that price.