There is one thing I would check about the hiss issue. I too heard a tiny, really tiny, hiss at max volume. But if you guys have Audyssey On, the hiss amplitude can depend on the Eq curve that Audyssey applied on your speakers. If it found the highs of your speakers are too low for,your room, it might have applied a big boost at certain high frequencies that could over amplify some digital noise or analog noise present in the circuitry. I'd say that if you still hear the hiss with Audyssey off and no Eq of any kind, THEN, it might be an issue. But if you only hear it when Audyssey is engaged, then it might only be a result of the Eq curve that Audyssey computed. In this case, everybody will get a different result because their room and speakers are different. And again, I would not complain about that if you can't hear it when you are more than an inch away from the tweeter, which is the case for my AVR-4520 and my setup.
Just my two cents on the subject.
Anyone who suspects this might be the case can inspect the corrections being applied by Audyssey by going into the menu and selecting Speakers/Audyssey Calibration/Check Results/EQ. A normal calibration should show a gradual roll-off in the high frequencies. If the EQ curves show an increase in high frequencies, it is a possible indication of something wrong that Audyssey is trying to over-compensate for. It might be worth looking at.