Are you hooked up with cable TV? If you are, disconnect the feed and listen again.
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I would say that when you don't hear the "hiss" from the receiver and hear it from the receiver/amp combo it is just the added amplification. The hiss probably present in the receiver but not noticable until the amp amplifies it.
Also it may be a RFI/EFI in the cabling between the receiver and amp that you're picking up. Try a sheilded cable