Howie brings up a good point. If ONLY a tube is bad (as opposed to a fuse or resistor in the audio board which took out a tube), then it could explain the intermittent noise and why it gets better over time. Clearly, it is in your amp as that explains why the noise occurs even when the preamp is disengaged and stops when the amp is in mute.
If it IS in the audio board, you will hear a distortion in the treble region especially when acoustic piano music is played.
One thing you could try is to get some spare 12AX7s (the tubes at the front of the amp) and swap them out. These are easy to find cheap tubes. Or you could also buy new 6H30s and/or the other tube (I think it is a 6N6P) and replace them all. If it is NOT the tubes, at least you now have a spare set.
It may be worth a try.
If it IS in the audio board, you will hear a distortion in the treble region especially when acoustic piano music is played.
One thing you could try is to get some spare 12AX7s (the tubes at the front of the amp) and swap them out. These are easy to find cheap tubes. Or you could also buy new 6H30s and/or the other tube (I think it is a 6N6P) and replace them all. If it is NOT the tubes, at least you now have a spare set.
It may be worth a try.