How to avoid hum and hiss to speakers from amps.

Two friends had been complaining about a hum through the speakers. Both problems solved with a power conditioner.

Judging from the responses, I'm wondering how much hum and hiss we're talking about. Volume turned up full I can hear the tweeters buzzing but not much at regular listening levels unless my ear is near the speaker. Is it the complete absence of noise or just getting rid of what can be heard at "normal" levels that's trying to be avoided? I have always lived with a bit of noise but never imagined that it could be gone.
My earlier post was for an annoying ac hum that could easily be heard at regular listening levels.