It is some BS. There is not a "sonic difference" between amps and AVRs for the most part. If you have enough power and the amp/AVR will handle the presented load, it should all sound the same.
I have 10 bucks saying that this dealer doesn't carry Yamaha so you bet...the Marantz, that he does carry is going to work much better...
That being said, the bigger paradigms do like alot of power but adding a amp to the Pioneer is the way to go about it.
I have driven my 100's with an AVR, an Emo amp, a pro amp and a Rotel. The 3 amps all sounded the same, the AVR didn't have enough balls to push the 100's very hard. Keep in mind, my room is huge (~8,000 cubic ft).
Try to drive them with just the AVR first and see how it works, add an external amp if you need more power after that.