| Mesa/Boogie M-Pulse 600 high-frequency hum
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My M-Pulse 600 has recently developed an annoying high frequency hum when it has been on for about half an hour. I've checked cables, bass batteries, speaker cables, and nothing seems to fix it once it starts. Turning down the volume of the bass does nothing, which leads me to believe it is an amp problem. I have also tried turing off the amp and turning it back on shortly after but the hum persists. The only way to kill the hum seems to be if I turn the amp off and let it "cool down" for a while before turning it back on.
I bought this used in 2009 and I have never had it looked at or re-tubed so I figure I'm long overdue anyway. If anyone can shed some light on this issue that would be great. Is it a tube problem or do you think I will have to dig deeper?
I live near enough to M/B headquarters in Petaluma that I've been thinking I should bring it by there anyway and have it tuned up, but I'd like to get a feel for what is wrong with the amp and what a likely fix will be before doing so. | 
