Well.........
Terry sent me a replacement 6L6, and after 2 sessions with 6 hours combined playtime, no issues whatsoever. Same old yummy sound that these have always delivered. I am watching them closely, and never leave the room when playing, just in case, but so far, all seems well at this point.
Terry suggested that it could have been a weak small tube or mismatched triode sections in that last 12AX7, or a poor connection in socket on that tube - so I reseated those.
Not complaining by any means, but I've always been less than comfortable with a fix where the ultimate cause is not determined.
This reinforces the fact that I need a tester, so I could rule out or include some of the above theories, at least where tube strength and section matching are concerned. I find this aspect of tube ownership to be the most frustrating.....no way to verify anything regarding purchased tubes - as I don't want to test tubes "in circuit" - and for troubleshooting in situations like these.
I have a LOT to learn.
Number 9:
Do I EVER hear you regarding those 6U8's. I've had 6 of these, supposedly tested, and 3 didn't cut the mustard. One worked good in my MX-110 for a week, then went bad, as I began getting flabby, distorted, loose sound from one channel. Thought it was an amp, but when reversing the channels, I discovered it was one of the 6U8's in the pre.
I am told that this is not unusual where the 6U8 is concerned. This is where a reputable, satisfaction guaranteed tube dealer would come in handy.