Hi Rick and all,
Ground loops produce low-frequency hum at 60 Hz or 120 Hz. Hiss sounds like pink noise at low levels. I think you may have a level adjustment problem with the surrounds, although I should point out that on older surround receivers, many manufacturers used to use lower-quality amps for the surround channels as a cost-cutting measure. Some of those could be noisy. This was true in the early days of analog Pro Logic receivers, before Dolby Digital 5.1 became a standard. It has nothing to do with the impedance of the speakers.
In a well designed preamp/amp, thermal transistor noise (hiss) should not be audible unless the volume control is set very high and you are standing fairly close to the speakers.
Preamps with extra stages of amplfication (magnetic phono, or moving-coil phono) may have audible background hiss at normal listening levels if it's a cheaply designed phono stage.
Axiom Resident Expert