It sounds like your gain settings are normal. Remember that you can use the loop return knob on the back as a global master volume. It comes in handy with an amp this loud.
If you set the loop to insert, the Return level knob is what you adjust, just like you said. I did not get a chance to play with it as I had Christmas leftover stuff to do Am I going to jumper the send and return or is the level adjustable by just selecting "Insert"?
As far as Master Volume goes, I am admittedly a noob. I think Would using this control as a global master mean I would crank the individual channel volume levels and control it overall with the Return level knob? Lots of questions, I know.