Hi SteveD!
You know I love your amp! I just picked up my second JMP1. I bought it to have two with different mix tubes to get variations of the sound I love in these babies. I haven't change tubes in any of them yet (in fact I haven't bought the tubes yet!) when I compared the two there is a considderable difference in brightness of the two. They are from the same batch and one of them I had for about two and a half weeks. The new one is very much brighter and I have to roll of the treble quite a bit. The "old" one is much darker and not as clear as the new one. When I play these I play them hard. The change in sound is quite different much more dramatic then breakin in a 50 or a 100 watter after tube change. Will this change continue and what is the expected lifespan of tubes in these amps? What tubes are stock in these amps?
Best plexi