Within each genre it becomes LESS subjective.When you cross musical styles or Eras in time it becomes more subjective. 40 years ago, there was NO " Crunch" in country but there definitely is today, even on the chords.
From an Audio Mastering point of view or an audio engineering point of view which you've touched upon and for example-in a Trad Jazz CD ,"stepping on" the other instruments in the mix is very different from a modern Metal CD with guitars going down to B instead of E.
Likewise with how smooth or square wavy the distortion can be on the guitars, the amount of gain etc. and the amount of space in the Mix which can be occupied by the guitars and the Rhythm guitar etc.-these would differ VASTLY on a Big Band Jazz with guitar CD,on a 3 piece Rock Band, a 7 piece country band, a metal band with 2 guitars etc etc etc.
The amount of what some people call "graininess" is another thing, even with a medium amount of gain you won't hear much "grain" on an Eric Johnson or David Gilmour recording , hard rock more is allowed, metal even more is acceptable ,right?
So it's not scientific and you can always " stretch" the allowable tones- especially if you're very talented but there are certain (vague, but still there) standards for what is good or acceptable guitar tones in each genre, I think.
