Just joined Talkbass and am doing initial post. I have looked at the site a couple of times in past and it seems there are quite a few members with a depth and breadth of expertise. I am hoping some folks here with greater knowledge than mine can add some info or insight about a problem I have with a bass amp. I have a GenzBenz Shuttle 6.0 that I bought used a few months back and have just had an occurrence that is unprecedented in my experience: when I turn up the Mid pot past 3 o'clock (and this is the only pot that has the effect), a sound that is exactly like a microphone feedback howl occurs and the blue limiter light goes on/flashes. This is a continuous noise and stops as soon as pot turned clockwise past 3 oclock. It occurs in all cases - either something plugged into input jack or not. IIt may have always done this, but i haven't noticed it before, as I had never turned up that pot that high before, that I recall. Some things I had recently done to amp: 1. I had replaced the 12AX7 tube with a new one about a month ago (sovtek) 2. I had used Deoxit spray in all pots to clean scratchiness two days before.(I discovered this phenomenon when I started turning dials to show someone how much of the scratchiness had been eliminated the next day) The amp works as normal when Mid pot is set less than 3 o'clock. As stated earlier, no other pots are affected the same way. Any ideas that others here have that may help in diagnosis would be welcome, especially those with GB knowledge.