I once asked this question from, Bob Cordell of Cordell Audio at www.cordellaudio.com.
Here is his answer.
"The hiss you hear is the natural noise of the power amplifier. All electronic cicuits make noise, some more than others. When you put your ear to the tweeter, that is a very sensitive test for noise, especially if your speakers are sensitive speakers. It does not mean that there is anything wrong with your amplifier, especially if you hear about the same amount of noise coming from both the left and right speakers"
Here is his answer.
"The hiss you hear is the natural noise of the power amplifier. All electronic cicuits make noise, some more than others. When you put your ear to the tweeter, that is a very sensitive test for noise, especially if your speakers are sensitive speakers. It does not mean that there is anything wrong with your amplifier, especially if you hear about the same amount of noise coming from both the left and right speakers"