TLDR: Look for a board mounted component that has developed a weak connection to the board.

Circuit board tolerances can be very small and once a connection gets weak or sketchy, even the slight amount of board flexing and taps as you change tubes or cart the amp around can be enough to restore/revert a bad connection.

I had something similar, perhaps?, going on with a peavey rockmaster preamp. A chronic problem where it was losing about half the gain intermittently and randomly. What fixed it was unplugging and forcefully reinsterting the cord at the jack in the rear of the unit. So I assumed the jack was dirty and cleaned and cleaned it and cleaned it.....to the point where I was sure it wasn't the jack.

So next I did exhaustive comparisons of lots of different 12ax7s and got contradictory results. This tube sounded great 5 minutes ago, now it sounds about half as strong at the highest gain settings.

When I was sure it wasnt the tubes, I used a pencil and gently tapped some of the larger board components starting with the relays and as soon as I tapped one, it clicked on and the thing roared back to life. The problem was never the tubes or the jack; it turned out to be a bad connection of this board mounted relay that switched in a gain boost via a pull pot on the front panel.