You can wire up a 1/4" input where one of the XLR inputs is.
Buzz and hum could be a bad ground or filter cap problem.
An easy master volume can go after the tonestack, before the phase splitter.
Or a post phase inverter master.
Of course the tonestack values should probably be changed for guitar use.
Dig up some simple Fender or Marshall schematics....they'll point you in the right direction.