OK now this thread is getting interesting! I really need to ask for clarification on something: is this thread about amps or amplification in general? It certainly seems that some people talking in this thread seem to think that these are the same thing. In fact they are quite different. Then there seems to be another misunderstanding - that "Pro amp" means the same as "cheap amp". I personally own no cheap amp but do own a Pro amp - a Bryston 7B ST which I picked up used earlier this year. I don't think anyone on here would call it cheap but yet it is often used in Pro applications such as recording studios.
By nature, the amplification built into a receiver represents an engineering compromise. To be competitive the receiver can only weigh so much, be so big, and cost so much. So the amount of heat that can be dissipated is only so much, and the parts budget is limited, and so on. This means that the amplification capabilities of the best receivers are simply not as good as the better separate amps. In fact it shouldn't be surprising if all but the worst separate amps are as good as or better than the amp circuits in receivers.
I certainly found this to be true when I compared my Denon receiver to my ADA 1500. The ADA1500 simply blows away the amp circuits in my Denon. No contest!
Comparing different separate amps like the ADA and a Bryston or Emotiva, however, would be a much more interesting exercise. However it is unlikely that a cheap amp would hold its own in such a test. Nor would any receiver.