#2
I've had a noise similar to this, though the source was in my poweramp section, but it was a 'preamp tube' (a 12AT7 phase inverter) replaced it and the problem was gone!

Sounded to me like a crackly rippley sound which didnt stop or change although it did eventually get louder over time as the tube was going
Mesa/Boogie Studio Pre | Marshall EL34 50/50 Power | Harley Benton 2x12 V30's

How do you like, your mids in the morning?
#4
Yeah, I would assume its a preamp tube, but what I mean is from my experience it was the phase inverter tube so give that one a try first