I do not see it as "subjective." There may be a family of adjectives involved and not everyone agrees about what are the most important. That said, one might consider the following:
Instruments positions are spread out and in definite locations.
(Opposite: they come from everywhere or are only vaguely "right" or "left", etc.)
Instruments sound like themselves -- an oboe sounds like an oboe and not a clarinet, etc. Moreover, they sound like particular, individual instruments.
Small details in the mix can be heard because of the separation and, as mentioned, the low noise floor. Test you can try: compare a complex track on two systems; the one that contains "more" in it is the more resolving one.
None of that is subjective. It is clearly perceptible. And perception is an objective fact in the world.