I just got up this Vampire Head from it's second owner who says it just started to hiss recently.
There doesn't seem to be any correlation between any settings changes and all the rotary control knobs and Master Volume pot are functioning normally and doesn't make noise when adjusting. So I doubt it's a dirty pot. I opened the unit and could not see any damaged components. The Mosfets and Preamp and all other board components, wires and connectors look pristine. (No Swelling, Discoloration, Heat Damage of any kind... Looks brand new.
I've been looking for schematics to no avail and have spoken with other audio techs familiar with guitar modeling Amps & PreAmps. One suggested that it might be the compressor malfunctioning, suggesting that it's stuck on. I'm not convinced that that is correct because the compressor function knob seems to change (+/- compression) but not stop or alter the hiss in any way. It's just there even before you roll in the volume pot on the guitar.
It's there as soon as its powered On and doesn't change at all even when all circuits are closed with no input on the strings. Then the second you strum it, even softly, the hiss vanishes for as long as you allow it to sustain... As soon as you mute the strings, the hiss comes back.
Any leads on schematics would be appreciated and I am open to any related product anecdote that may lead to an answer or an outright solution from first hand experience. It might not be from abuse or pushing it beyond its specs or ability... It could be as simple as static discharge damaging an amp Mosfet(which can occur at the output stage when connecting a cab or ????).