OP, bravo for coming up with this scale. I like it and will try it out.
You're working out ways to express gradations of improvement.
That keeps us organized in terms of expressing degree of change. Helpul!
Also need to stay organized about:
(a) what specifically we are listening *for* (treble, bass, overall sound, how voices sound, pace, etc.) Here the vocabulary list from Stereophile is very helpful.
(b) What other biases might be at work in our process — mood, time of day, money spent, recording, etc.
Cross referencing the scale of gradations you've listed plus (a) and (b) creates a good way of keeping ourselves organized to make judgments. I'd love to see a scoresheet with all these factors in it.
Because this kind of task is complex — and room and ear and taste dependent, to boot — there's virtually no way to compare what we hear with what others hear, with any fine-grained accuracy.
Still, if they're in our room with us, and a number of factors above are controlled for, we can find common ground. Lots of people stand in front of paintings in museums agreeing about shared observations. No reason it cannot happen with sound.