I read that too - guess it does sound more like a buzz than a hiss... though 60 Hz would seem a bit low but can't really tell. My sound meter did pick up the buzz at about 55 dB.

In that link it mentioned that ground loop can be caused by too much hooked up, so I unplugged everything except the amp - and the buzz did not stop. Even plugged it into different outlets and still got the buzzzzzz.