RetroTodd, welcome to Audiogon!
I haven't ever heard an MC-225 or an 8B. But I know that the 7591 power tube used in the MC-225 is a fine tube, and I'd imagine that it will prove to be quite a nice sounding amp, especially with the substitutions you'll be making.
I did own a pair of Marantz 2's, however, about 20 years ago. As you indicated it is an incredible amp. Around the same time I also owned a pair of the legendary (and these days ultra expensive) Marantz 9's, and the 2's easily outclassed them, especially in triode mode (when paired with speakers that were suitably matched to the relatively limited power capability of the 2's, of course). Although the condition of the particular amps could certainly have been a factor in that difference.
Here is a direct quote about the Marantz 2's from the system description thread of one of our most knowledgeable members, Kirkus:
The best of the vintage amps. Really. ...Sound is clean and detailed, not bright but not at all mushy. Incredible dynamics - better than the 500W/ch solid-state amp they replaced.
What I was most struck by with my 2s was their richness and accuracy of timbre.
Regarding a comparison between the MC-225 and the 2's, while the following facts are certainly not conclusive of anything they seem worth reflecting upon:
According to information at roger-russell.com, the MC-225 weighs 34 pounds, and cost $198 in the 1960s.
According to literature I have, a pair of Marantz 2 monoblocks weighs a total of 94 pounds, and cost exactly twice that amount in the 1950s, and $440 in the early 1960s.
In any event, assuming its in good condition Im sure youll enjoy the new amp.
Regards,
-- Al