I have had a similar issue. Not so much a hiss as a hum. This only when I had my HD500 plugged into the computer via USB. I also found that computer sounds were coming through the POD and out the amp. The only thing I can figure is that somehow the computer (Dell running Windows 7) is seeing the POD as an audio interface and sending audio out that way instead of the normal way (Steinberg CI2).
That may be entirely different from your issue, and I haven't bothered trying to fix it either. But just in case it IS part of your issue - there you have it.
