Steve-
The Master Volume is for the Monitor Mixer and will only be active if you assign the 1/2 output to the Monitor Mix on the Patch page. At that point you should be able to adjust the Master fader, but that is only goiong to adjust the output, and in the single output configuration of the 192, it is really a moot point.
Tell us how you are routing your signal through you mixer, where the gains are set, and how you've got everything cabled up. It sounds to me like you have a gain staging problem somewhere.