Not too sure there is any difference from the processing stand point of an AVR vs. a separate pre/pro and most avr's are at the bleeding edge of the tech while separates seem to lag behind.
I do feel adding an amp does help clean things up sound wise at higher volume levels and I have found the lower frequencies are played with more snap/punch. Whether or not these differences are worth the extra dollars spent is upto each individual. I am quite content with my Denon 3808 but will eventually add an amp at some point.
