It’s not a master. There is a normal and bright channel and they both have a volume knob. The cool thing with the 5e3 is the unused channel still affects the channel you’re plugged in to. Then the tone knob acts as a kind of gain/dirt control in addition to altering the tone. This is the magic of the 5e3 and how you get all the different tones out of it. You said the owner modded it to make the volume knobs not interactive, this is why it is doing nothing. This is the one mod I would never do to a 5e3. Google how 5e3 volumes interact and you will find several videos showing how it’s supposed to work.