Embarrassingly enough, I was able to recreate the problem again. It was when both NORMAL AND BRIGHT channels were over 12 noon, even when I was not using a Y cord. Something about the 2 gains at the same time. When I used NORMAL or BRIGHT on their own, with the other input volume low or off, there was no issue. I probably wasted everyone's time AND my money blaming the amp when it was probably not even the tube socket at fault. It never occurred to me,(and I don't recall) the settings I used on the live gigs BEFORE when the "issue" occurred.
So in some way, I get resolution, knowing it was not the amp's (or Reeves') fault, nor was it mine, aside of missing the "conditions" that caused the issue.
I didn't get back to Bill with this because I discovered it just a few days ago and I felt that I bothered him enough.
Lesson Learned!!
BTW, this amp completely blows my mind- how good it sounds AND feels!