I'm definitely no expert but, from what I can remember from my science classes, you might be hearing noise floor, which is is the totality of all unwanted signal generated or received by the system. There are a few basic things you can check for reducing hiss:
>> cable connections and terminations: Are they clean, neat, trim and tight
>> interference/shielding: Are your cables and speaker wires close to other electrical sources or fields; can they be moved or shielded?
It's also possible that it's something you're stuck with. My parents' old system used to hiss. Only time I paid attention to it was in the quiet passages of listening to western classical music or when the system was first turned on.
Have you tried hitting people on the Audio Theory board here?