The problem with removing one of the jacks and putting the master volume there is that the circuit should always go in as straight a line through the amp as possible, and the input jacks are on one end of the chassis towards the preamp. So if you added a master volume and put it there, then the sound would loop back on itself from the preamp, through all the circuitry as it moves down the chassis, and then back to the preamp area, and most likely would cause oscillation issues. The proper place to put it would be down further in the middle of the chassis past the tone controls. Unfortunately to do that would require you to drill a hole, or to move every one of the other controls over, leaving a space for a master volume knob. A better approach than the master volume is power scaling or VVR, but again, those would require holes to be drilled. If it is your amp and you don't care about any collectibility associated with it, then do what you want. If you do want to maintain resale however, then the least drilling of holes is adviseable. You could hook one up outside of the chassis on a temp basis and see if you like the effect. Be careful if you do this however so that you don't accidentally shock yourself when you go to turn the control. A typical master volume such as what Marshall traditionally used right before the phase inverter would work, though it would work much better if you put a 100k resistor in between the output of the master volume and the input of the phase inverter.
Greg
