With high gain the upper harmonics are super important. Dialing back either the tone or volume just seems to flatten those harmonics (though sometimes dialing back the volume just a hair is a good way to take off a sharp edge when you want to.) When I was learning to play I plugged my S300 into some no-name boost/fuzz pedal and then into my Princeton Reverb which I had modified to use the 'vibrato' tubes for more gain. It sounded like it was about to explode and I was always reaching for the volume and tone knobs to make sure they up all the way.
But, in recent years I've been wiring more guitars "
50's style" and find that there are a lot of good (not muddy) sounds with both knobs rolled back. I think this is especially true with P-90s where a light and airy - but still bright - sound can be found. Interestingly, I usually find that sound with both knobs at about the same place: both on 7, or both on 6...
Different people feel differently about 50's wiring, but I know I always left all the knobs on 10 before I started wiring guitars that way. Now I use them all. Could be coincidence. Could be the style of music I play has changed (less gain.)