Hiss and not hum? I've got a B&K Ref 31 - a nice preamp - and I have a low hiss that you can only hear when you move up close to the speaker. Some of what you are hearing may be the noise floor of the Onkyo receiver - although hearing it from 10 feet away is not right.
I read recently that "hum" is a ground loop situation where "hiss" is an EMI/RFI problem. Do you have a power cable running parallel to your surround speaker wire at any point? Someone else may want to chime in as to the causes and solutions of EMI/RFI in a system. Like I said, some of what you're hearing is noise floor, and some of it is the inherent nature of the rats nest of audio interconnects, speaker cable and power cable running behind your system.
