I initially thought the valve in V1 had gone (or one side of it, at least) but after changing it out I still have the same problem.
I noticed it for the first time at rehearsal a couple of weeks ago. The clean channel was just really quiet. I figured my dials had been moved, but they hadn't. Master volume in the same place as it always is. Everything the same.
When I flick channel 1 from 'clean' to 'pushed' the volume all of a sudden leaps back up. Those two modes are (or were) the same volume before, they just had a different voicing (obviously one being clean, the other being a dirty clean, blues break-up kinda tone).
The rest of the amp is absolutely fine and there's no change in channel 2 or 3.
1.) Change *all* the preamp tubes until the problem goes away.
2.) If it doesn't go away, look in the chain in front of it (if there is one.) I had a goofy issue like this and it turned out to be a ground problem (bad jack) in the looper I use for my Tubescreamer.