I own the 8100 but can answer your questions:
Yes, there is a difference between the two. The 9200 sounds a bit more dynamic due to increased power supply. It is a fine piece of equipment especially at the used prices on the 'gon.
Yes, the 8200MkII has the processor switch. You can feed the inputs from a surround processor (thereby bypassing the Plinius' pre-amp stage) or run a loop through an external processor. I would think the loop would allow one to send output to an external crossover (e.g., powered subwoofer) and the signal would then come back to the Plinius.
Speaking of low prices, the 8200 should be a screaming good deal now that the 9200 has been out in the marketplace for a time.