"Has anybody compared the 252 to any of the “top dog” solid state integrateds from Mc, such as the MA8900 or 7900 or 8000, etc.?"
I went from an MA252 to a MAC7200(200wpc).
The increase in power really creates a more realistic listening experience, IMO. Bass and drums now have a "weight" to them that I didn't have with my MA252. Also, the three-dimensional imagery is now equal or better.
I loved the MA252. Fantastic piece for the money. MANY recording playbacks were "best-ever-in-room" and previous integrateds include the Bat VK3000se and a Cary 300SEI among others.
I did have a hard time finding a speaker match for the MA252. I did not like Focal 1028BEs (way sharp, probably the be tweeters), and the amp could not drive McIntosh or ATC offerings without a lot of "orange" (clipping) happening. It loved my current speakers, JBL4429 monitors.
Ultimately, if the MA6500 behaves similarly to the MAC7200, you (OP)really won't be in a better world from a warmth, richness, sound stage presence standpoint. But it does look cool!