I am not an expert on the A500, but I read your question differently than how most answered it. Seems like you want to know why people use a power amp combined with a receiver.
The point of the added power amp is to drive the main speakers, and let the receiver drive the surround channels. This only important when you have big towers for your main speakers. Anytime you are reproducing bass frequencies (as most large mains try to do), you are using way more power than the receiver was designed to provide. Ever ran a subwoofer off a receiver? it doesn't work very well and that is why subwoofers have 300+ watt amps while small bookshelfs can sound great with less than 50 watts.
If you have your speakers set to small and the xover at ~80, there is no reason to run big power amps for your mains, athough I'm sure someone here would disagree.
The A500 is popular cuz its gives a lot of bang for your buck. But if the distortion problem continues it will prove all those pro-gear haters were right, and that behringer's cheap products are exactly that.