Swap the two speakers and see where the hiss goes. That will tell you if the spkeaker itself is to blame or amp (more likely). As others said, if you can't hear it from your normal position it is not a problem.
How Much Hiss Is Normal?
I have some Quad 12L Actives that I'm still setting up. I notice an ever so slight hiss from the speaker even when it's not connected to anything. You have to be right next to the speaker to hear it though. The volume setting does not change this. I'm reading that this is not uncommon for (other) powered speakers. They are both connected to a power cord surge suppressor. I assume this is normal??