well i don't know if this is true for sure or not, but from what i have gathered and read, the more gain changes, up or down, it doesnt matter if its decrease or increase, you get more math errors, more data loss, and it adds up. if its just the main fader that gets the gain decrease instead of all the channel sliders, it makes more sense to me that becasue more gain decreases from all the tracks would add up more errors than just turning down the main one...
let me know if i am way off, i could be, that is just what i think so far from what i have read. because the more db you change over all, the more damaged the sound becomes..
and seriously, when i leave my channel faders alone, or turn em up a bit, and crop the main/master, it sounds louder, and more present, like there is more volume to the sounds, when i turn down all the channels, and turn the master up to 0 and mix that way, i think the sound sounds more distant.. more plastic like... just from my ears in cubase, it could be just cubase too.. i dunno how they do the math for their mixing pre master fader...
i havent done any science to prove or disprove this, its just what i hear...