I'm new to this site and I'm hoping someone can help me out. I have a Mesa Boogie Bass 400+ and I recently experienced a loud buzz at rehearsal. It was intermittent but got worse with time. It would originally only happen after an hour or so of playing and then go away after I would flip over to standby for a few minutes. Eventually it progressively got worse so I figured it was time to retube it. Well, I retubed the power section with 12 JJ 6L6's. It worked fine at lower volumes but when I turned it up to a decent level, Same buzz, exact same problem as before. My bass can still be heard but the level is much lower and the buzz is predominant over the Bass guitar and it seems to come and go. The funny thing is iwhen it isn't buzzing, it sounds great, clean and plent of headroom. I took it into a local tube amp guru and he's scratching his head. He played it for 1 hour yesterday at a fairly high volume with no problems. Today it started buzzing after 15 minutes?????? He's thinking a grounding problem???? Any help or experience with a similar problem would be greatly appreciated.