You're using a single input aren't you? Don't feel bad, it had me confused at first as well.
That's your gain control. And like a solid state input section on a multi-channel amp you can't adjust gain separately if you only use one set of inputs.
Once you set the gain you use the level control on the bottom right (over by the bass boost control) to set the individual channel or channel pair levels.
That's your gain control. And like a solid state input section on a multi-channel amp you can't adjust gain separately if you only use one set of inputs.
Once you set the gain you use the level control on the bottom right (over by the bass boost control) to set the individual channel or channel pair levels.