It is often said that the electronics in seperates are of higher quality and offer better performance, ie faster attack and quicker response, other than that there isn't much difference as anything below 1% THD is unaudible, which most avr's and seperates are rated at. So far as most avrs not rated with all channels driven, well that is wrong, I agree some are rated this way, but most of the ones mentioned on this forum are measured with all channels driven at least as far I can tell from independent tests you can find on the internet. Some even exceed their ratings.
