No guarantee that this will fix it, but I have a few Ebtech hum eliminators. They are line level isolation transformers. They won't fix every hum, and in one case it introduced a hum, but they've worked for me more often than not.
Other than that, I would check your mains outlets for open ground as a start.
Good luck.
-n
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Re: [Doepfer_a100] Possible Ground Loop - help needed
2008-02-11 by neil jendon