I have met 2 very successful prospectors now who always run their GBPros at full gain, even in some pretty nasty ground. I honestly have no idea how they do it; my brain turns to MUSH listening to that much chatter and erratic threshold... but they can do it and are successful so you certainly can't say that it's wrong. And yes, with the gain up all the way they can hear those tiny bits even better. I just can't handle the wild threshold personally.
I think most people, myself included, are generally running around the 10-11 o'clock position.