well, I'm not a current user, but had the 750.
the gain definitely has an effect on tone, when you add it, it sounds thicker, more mid-focused and saturated. I usually add gain, until my all strings are equally saturated, a bit more and the low B clips it, a bit less and it all sounds thinner.
also pay attention that when you dig harder, the preamp starts to overdrive (not very gracefully when driven hard), if you want a cleaner sound, just back up gain a bit.
a moment where those two moments meet is a bit before the light comes on, I find it roughly with the LED and then back up a little judging by ear. where's that on the dial? unpredictable, i.e. depends on your bass and playing style. I have three basses with very different output level, so I adjust the gain for the bass.