i may have already said this elsewhere, so forgive a possibly double-reply. but, my first experience with separate amps was with the exact amp you are discussing here. i used 3 audiosource amp one a's across my front 3 speakers (each amp was in bridged mode and 200 wpc into 8 ohms, claimed).
several times, i listened to the same movie scene over and over, switching back and forth between having my yamaha 2500 driving all 5 speakers versus the audiosource's driving the front 3 with the 2500 driving the rears. the improvement was not night and day, but it was enough for me to be glad i added external amps. movies seemed a bit more dynamic (especially in complex scenes with a lot of noise), the presentation of the soundtrack seemed a little more powerful, and the amps seemed to have more oomph and control over the soundtracks.
being happy with the movie side of things, but not so thrilled with demanding 2 channel music performance of the audiosources (they were OK, but sounded a little harsh at higher volumes), i decided i wanted something a little more upper crust for my front left and rights. so i added a B&K 200.2 S2 with 225 wpc to drive my front left/rights. now, 1 of my amp one a's drives my center speaker and the remaining 2 amp one a's drive my surrounds.
i am now even more impressed. my B&K gives the front soundstage a more powerful and cleaner presence than my audiosources. the audiosource's run in mon mode on the rears have made the surround effects even more enjoyable.
in short, i believe most mid-grade receivers do a commendable job on surround sound. but, external amps, especially if you can do monos or 2 channels, all around really give some extra punch. that being said, external amps are a completely unnecessary luxury expense, given the good performance of most receivers by themselves.
