Orpheus,
Music is indeed subjective. From the same performance you may get a boogie while I get a blah, or vice-versa. ;-)
It does indeed follow that adjusting a vinyl rig to make music sound "better" would be highly idiosyncratic, perhaps not even repeatable by the same listener if his mood changed from one session to the next. I believe that's your argument, right?
Many posters on many forums seem to do exactly what you denigrate but I do not and neither does my partner. We adjust WHILE listening to music but we do not adjust BY listening to music.
With native ability and/or practice, one can listen to and analyze sounds strictly as sounds, seperately from experiencing them as music. Read my posts going back many years. When discussing adjustments I never mention "musicality", "prat" or emotional/musical content of any sort. I discuss only sonic attributes like the shape, timing, amplitude and interactions of waveforms.
There's no knob on my tonearm to make anything more or less musical. There are several knobs that affect rise times, amplitudes, decays, sound floor, phase coherence, crosstalk, etc. It is these sonic attributes - not musical niceties - that I adjust for.
When I'm done optimizing the sound I may turn off my analytical brain and enjoy the music, though in fact I generally hear both simultaneously. I realize that some people do not, though from my own experience I believe that many people could.
Example: if I played an LP groove consisting of one well-recorded tap on a snare drum few people would call it music (apologies to John Cage). Yet I could optimize VTF and SRA for that LP by listening to that tap. It's no different when I'm listening to Brubeck's 'Time Out' or Stravinsky's 'Petrushka'. I can pick out and focus on snare drum taps for tweaking even as I enjoy the whole musical presentation as a seperate aspect of the same experience. I don't believe this is very different from musicians who often tune or tweak their instrument in the middle of a performance. They're feeling the music but they're also hearing the sound.
Hope that clears up more than it confuses!