What you may well be hearing is the amps' "noise floor." The higher the sensitivity of the speakers, the easier it is to hear it. Every component has a noise floor. Not that I'm an electrical engineer by any stretch of the imagination, but I think it's impossible to not have one. Some are quite lower than others, and some may be inaudible.
It could be something else, or a combination of several things. In my experience, hiss is usually amp noise floor. Humming and buzzing are usually something else - ground loop, EMI, RFI, dirty power, etc.
It could be something else, or a combination of several things. In my experience, hiss is usually amp noise floor. Humming and buzzing are usually something else - ground loop, EMI, RFI, dirty power, etc.