Wow, man , I don't know. Since you say you don't hear it in the tweed mode, it would seem like a tube problem since the line voltage is dropped to about 93 volts in tweed, so you're not driving the tubes as hard. You also mentioned that the noise increases as you turn up the effects loop send. The effects loop is in between the pre-amp and power amp stages so it seems like the problem is somewhere in the pre-amp section. (The effects loop acts as an additional gain stage so when you turn the send up, it's amplifying whatever it's getting from the pre-amp.) If you don't use the effects loop, you can just bypass it. I don't know what the odds are of having multiple pre-amp tubes going bad, but I just had V2 go bad which caused a sputtering/static noise in all modes. I got a replacement from Mesa and all was good. Yesterday I noticed a hum in channel 1 with the reverb turned up. I turn the reverb down, and the problem goes away. I replaced the V4 pre-amp tube (Reverb return/drive) and the problem persists. I know this doesn't help but I can understand the baffling feeling you have. At this point, I would give Mesa customer service a call. Let them know what you've done to troubleshoot the problem and hopefully they can provide some ideas beyond sending your amp in. Good luck man, and keep us posted.
