I empathise with many of your points Veiko, having mused over what is going on in general with my involvement in Hi-Fi and music.
Not only do our ears change, as a result of, (and I have substantial) hearing loss, but our brains change; most of hearing is brain processing, and as a result is learning based.
One can assume that one's ears change little in a an hour or day, and so they are considered relatively constant, but when we change equipment we may hear a difference and wonder which is more correct. Often neither is so, but just different; our entire nervous system is configured to perceive changes.
IMO) amplifiers show far less difference than other parts of a system, particularly of course speakers, and it may be possible that a better amp shows up a failing with a speaker producing a result which is less pleasant.
I now have a system with what I consider to be the best tweeters I have heard, and on R2 a little while ago the top was excruciating.
I'm sure it was a combination of FM and the recording. (my amps are Icepower except for above 2.5kHz, and is then AB).
When we listen to music we hear what is objectively present to a large degree, but what gives us the great pleasure is what we ascribe, associate, assign, and attribute to it. We project onto it our own emotional empathies, and our collections become a repository of our projected emotional responses.
When I first realised this I thought that we were all being conned because what we experienced as great pleasure was actually based on our life experiences, and in our heads, so why should we pay for it.
Because it informs us of our own true self, much of which hidden and may be buried deep in our minds, often due to partitioning, compartmentalisation and repression; it tells us about our inner self, it is a catalyst to self discovery.