I've tested everything already, and no hiss, and no hum - even at moderately high volume settings (say, -18.0db).
Before getting everything purchased and in place (last January), I paid quite a bit of attention to others with similar plans, speakers or amps, and noted every time someone reported hiss or hum, and what they did to cure it. I was paranoid about that before I started, but as it turned out I didn't need to fret about it because all is dead quiet.
I made sure power cords and source interconnects, etc. were separated, and the coaxial cable TV feed was properly isolated and so on.
The Emotiva XPR series of refernence amps have a gain of 29db so using the THX standard helps too.
The JBLs are rather efficient, but no hiss from them either.
I lucked out, or just was very careful in set up(?).
BTW, the Emo is plugged into its own dedicated 20 amp circuit/outet, as is the AV8801 and all source components into another.
The Panny 65ST30 and right side subwoofer share a 3rd dedicated circuit/outlet of 15 amp rating.
Here's a shot of the right side: