I have a problem on my Ceriatone Liverpool with what sounds like a low-mid feedback when the master volume is past 4. It's sounds most excellent and functions normally until that point. I am using a 1 meg pot (trainwreck type 4 ppi). Any ideas on what may be causing the problem?

Oh may be the gain increases, try to less the gain of your amplifier,  try the Liverpool 4 EL84 cathode bias SS rectifier of 35w. May be your problem will solve.