Well Playa since this thing don't wanna go away, it's time to start narrowing down the possibilties, swap these out in order if you can, Room/outlet location, guitar, cable, pre-amp tubes (one at a time), that's about all that can be done at the user level, occasionally power tubes will hum when going bad but it sounds to me that you have it narrowed down to the pre-amp, I would even venture to guess that it's one of the middle two, you have four right?..................