ground loop would cause a 60hz hum. 60hz is the rate that AC "switches" so thats why its that.
The hiss your experiencing sounds like just plain fact that your reciever isint a top of the line model and its got some hiss noise. As alan stated, most good amps and preamps wont have this, unless you put your ear literally right up to a tweeter for example. I had a hiss which was audible in my old cheap reciever, its a way of life.
I dont see any reason whatsoever why your cables and their lengths etc would be causing this, but i would also think that you would be hearing the hiss from all of the channels if it were as bad as it sounds. Good speakers will reveal lesser equipment upstream, lesser media for that matter too. Ive been noticing distortion in movie soundtracks which is somewhat annoying.
