I'll be honest, I have a REALLY REALLY REALLY good feeling that this knob is the "remote gain." In which you turn the entire way up, set your gains with either the dmm or o-scope, then turn back down. Therefore the knob would limit you to only your maximum gain set.
WHICH, would not be using your "bass boost." Often referred to as the same thing, "gain" and "boooooooooooooooooooosssssssssssssssssssssttttttt" are different.