RD50 suddenly sounds anemic...any ideas?

Hello, esteemed fellow Music Man fans. Has your amp tone ever gone from the clean, ringing chime you expect to hear to a thin, anemic sound? I have a 112 RD50. Last week, it was frying out the power tubes. I took it into a shop and was told that leaky drivers and burned resistors were the culprit. I had them replaced, put in new 6L6’s and it seems to have solved that problem.
Now, when I play through it on the normal channel, as I usually do, it just sounds kinda sickly. You can whang an open G, and it seems a bit fizzy and diminished. The amp volume is only on 3-4, so it isn’t turned up enough to make the thing distort. It should sound clear and clean. I switched over to the distortion side and it seems worse over there. I don’t know very much about amp operation. Should I now try replacing the 12AX7 tube? That’s about all that I can easily do, but if that wouldn’t likely be the issue, I don’t wanna spend the money on a new tube. The one that’s in it doesn’t look burnt out or anything. I do realize it’s a 35-40 yr old amp and things break down/wear out, but what might this be? Any thoughts/opinions greatly appreciated!