Some high-power pro amps, Peavey I'm looking at you, have a very audible hiss even when not fed a signal. That isn't the case for many other makers, but is something to be aware of in amps which are designed first for power, without must though to fidelity.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris