B&K Sonata M200 + Polk Rti A9’s

i would expect a night-and-day difference, both because you m200s are more powerful and because they're  higher-quality amps than those in your avr, which have compromised power supplies, less robust transformers, etc. typically i'd expect more low end control and overall detail.

I have a pair of them (+ a single one I use as a center channel). I have had as many as 6 at one time. They are great sounding amps even ignoring the power and current output. I use them to drive Totem Mani 2's which themselves are very power hungry.

At that price you should grab them (assuming they are in good operating condition). Many of them out there are a little dinged up, and some of the older ones are pretty utilitarian looking, but they still sound great and are bullet proof.

Thank you everyone for the input but the sad outcome was that when I went to listen to them, there was a lot of hum in the background and I was afraid to take them since I don’t really know much about the issue. I offered to still buy them under the condition that I could try them at my house with my cables and speakers, and if that same humming was there, he’d take them back, he refused so I reluctantly passed.
now he’s offering me a Bryston 4B, which I know nothing about. 

The Bryston 4B would be a good choice if it is in good shape and working without issues! Yes it is an old amp but it is built like a tank and can drive anything! I have its 30 year old brother, the 3B - still going strong!

The m200s are also kind of old. My Nuforce STA 200 bettered them in every respect, even though the STA is less power and a single chassis. I gave them to a friend, who is going to do a complete rebuild. This was about a year ago, as the little Nuforce bettered many amplifiers in my collection, leading me to clean out my closets of equipment I was no longer using. In their day they were very good. No offense to anyone, just stating my experience ( I sold B & K way back in the day ). Enjoy ! MrD.