Keith, as far as I know there is little or no commonality between 150W 10-year old M10 and 300W M625, except that both operate in class A/B, and use some forms of SMPS.
I have no real information whether Rowland has "abandoned" class D or not. Fact is that at this time tech detail is a little scant.... All I know beyond the product page on the Jeffrowland.com site is that M625 and M925 operate in class A/B with some form of "variable self adjusting bias" (my term not JRDG's) and no "global feedback" (their term).
Technical commonality with M201 is even less, M201 being an entry level device and M625 far from it. As for M925, it will cost approx 10X M201... I sure hope there is some difference beyond chassis and price tag, or I'm going to be one unhappy camper when I finally get my own units (grins!)
As for the sound, I have not heard M625 and M201 in the same system, but purely from fickle memory, they do not seem to sound the same at all at all at all.
G.