I bought my 4001 in 1986. A couple of years ago, just because I never had the amp services—and despite it sounding like it was brand new—I took it to an authorized Marshall service center. The gentleman who worked on it told me all he had to do was clean the gain pot; that he didn’t have to change any tubes or do biasing or anything. Caps solid. He said everything was perfectly in spec‘s. When he was working on that amp I bought a spare. so when I picked the amp up from him, I only plugged it in once to make sure it was working OK and then I continued playing the new amp. Recently I began playing it again with the same exact pedals and settings I had in the past that gave me phenomenal sound. But over the last couple of months I’ve noticed something wrong, like every week or so I was losing more and more tone for lack of a better word. Then I noticed it seemed I was losing crunch. Long story short, finally today, after noticing that I had the gain on my OD pedal and the amp gain both on 10, I plugged my guitar directly into the amp. with the gain on 10 and the output level on five, and it sounded almost perfectly clean, no dirt. I get the same result whether the amp speaker is plugged into its 15W, 8ohm jack or in the attenuated headphone jack. Same thing. sooo... The power amp side master output level sounded plenty loud at five, so I’m assuming that side is OK. But safe to say somethings wrong on the pre amp side? any ideas? Thanks ahead of time.