Acceptable hiss ?

I would say, due all respect, it is normal for a receiver. I don't know much about your receiver, but it is common for those digital receivers to have relatively high noise floor. Try "direct" mode if available, and let me know what happens when you connect rear speakers to front terminals of the amp.

Justy, try to determine whether it is from the receiver. Connect right/left speakers to rear terminals, etc. If the noise comes from the same speakers, then, it's speakers or cable (or signal interference). But it is still possible in your case that it is normal, although I don't know how close your ear is when you hear hiss. In HT setting, you usually set the front two speakers louder than rears. That may be why you hear louder hiss from fronts. Anyways, in my case it was receiver and I think it's the prime suspect. BTW, If I don't hear it from listening position, I would not mind.