As far as you said this problem,it is probability the noise to cause.Especially power circuit is clean output!? If it has 50~60Hz frequency,then it will cause this problem.You can check this point first.
Does the hiss sound go away when you disconnect the input to the amp?
Was the amp good before and the hiss sound is a new problem?
Did you add new speakers with better or peaky tweeters?
Is the amp trying to whisper something to you? ;D