How to assess and quantify system improvement and changes?

My problem with trying to quantify this sort of thing is that in why experience the process is very non linear

By this I mean it goes something like ...

1. "My systems sounds great, can't imagine it getting much better (diminishing returns etc etc)"
2. Introduce component X or tweak Y
3. "wow, I never paid attention to/I was not aware of that (coloration, timing error, gross soundstage aberration etc) before - it's amazing how much better the system sounds without it"
4. continue tweaking to improve this dimension
5. Return to step 1 and repeat
   

So each step 3 can feel like a very large change, then followed by a series of smaller incremental changes and the large change is driven less by the specific quality of the change/new component that highlighted it but more by the fact that it became highlighted

Said another way we train ourselves to appreciate (and to some extent compensate for) the shortcomings in our systems, we "listen around" them and it's only when the need to do this is removed that we realize how much mental effort we were putting into doing this audio error correction ... ultimately then the best test of any change is does it make it easier to listen into the music and get to the intent of the artist