Silly question but have you tried a fresh GOOD QUALITY battery in the pre-amp?
From experience...when the battery begins to run down it can cause freaky stuff to happen...like a loss of tone or volume...haven't heard them cause THIS issue but that's not to say it can't.Man, Dothe battery influrnces the tone or squealing? When I installed the emgs 3 or 4 months ago, I put a new panasonic battery.