...I think hearing a difference is all over the map: sometimes yes. Sometimes no.
And a lot of maybes trying desperately to be a yes or no.
IMO one of the key issues: The premise is known: a change has been made.
There is a lot of "pressure" on the brain, ego and psyche to identify the change. A change was made!
Depending on your a priori mindset, there is likely a strong inclination to affirm a change: guys are right! This cable/box/thing is amazing. It's worth it! Guys survive for decades on a single kidney. Or...
to deny the change is legit. I don't hear it. It's not measurable. The website info is unquantifiable quantum gibberish.
I really think it's all over the map and there is a lot in what
@Ian B says regarding substantial changes, which do tend to be noticeable. But most of the changes we read about seem to me to be tinkering around the edges, and more difficult to accurately recognize.
But net-net: if you like it and you want it and you're not robbing a bank to buy it (what you do with your kidneys is a personal decision), then have fun.
I think a real pisser would be: have a hifi pal over and ask him what he thinks about the changes you made (don't make any). And listen to him ramble on about the sound.
Warning: You can probably only do this once per hifi pal, and some guys aren't coming back for seconds. Am I too cynical?