Scooter there is none..
Your talking about the gain controls on a GG. If you're mono and XLR, you have no gain control. If you're RCA and mono, you do. ONE of the pots will work it is marked "Mono" if I remember. BUT only to increase the volume to the same "max" as the XLR. Older MC275s, NONE XLR are different once again..
BUT there is no bias control for YOU to mess with. Mac self bias..
I own a GG Mac.. Actually on the fritz right now... My fault, though..
I had that one sounding pretty sweet until I did a cable drop..
$15,000.00 cable drop... That will never happen again...
Regards