Thanks a lot Ed! I had a few people say that you could mod the amp, and add an extra pair of Kt88's, so I guess they were wrong. I apologize. On the other hand, the MS is definitely a more grindy amp, and would work well for a multitude of heavier genres.
On the other end, would you lose any tone with using pedals with the Reeves? I wasn't sure if running them straight into the signal chain would disrupt any high, mids, or lows. I am trying to learn more about amps and EE in general, and I'd like to become more knowledgeable. I am not quite a beginner, but these two amps would be my first high end tube amp. I used to have a Mesa 400+, but sold it for financial reasons, and it wasn't exactly warm enough.
Theoretically you
could mod a C225 into a C400, but it would involve adding and/or upgrading components and would cost substantially more in parts and labor than buying a new C400 would, so while I am hesitant to say that the people who told you you could mod the amp are
wrong, I will say that they are "misguided".

However there is a common misconception that you can add power to a tube amp by simply adding additional output tubes, and that
can't really be sugar coated; it's just plain wrong...
It just doesn't work that way.
If by saying a 400+ wasn't "warm" enough for you means that it was too clean with too much headroom, the Reeves is probably going the wrong direction for your taste.
Having compared those two head to head IMO the C225 has
far more headroom than the Mesa, substantially more "hi-fi" and more articulate.
To my ears the Mesa had a "syrupy" tone, not real dynamic or touch sensitive, where the Reeves is bigger and bolder sounding with a huge soundstage and more sensitive to input dynamics.
Again; not a "good" or "bad" scenario, just
different amps.
Here's a
plus for the C225 though. It's
easy to make an amp sound dirty, not nearly as easy to pull off a clean dynamic with a lesser headroom amp; particularly in the lower frequencies. You could easily pedal up a Reeves to emulate that grittier Mesa (SVT, Meatsmoke, etc.) tone, but it would be pretty much impossible to get the same "big clean" out of the other amp's circuits.
It’s easy to
add distortion, but I’ve never seen a pedal or effect than can actually
lower an amp’s THD.
Along those lines, I actually have a C225 right now for “Alpha” testing that has been modded by the Reeves skunk works to “dirty it up a little”. It’s an ongoing project involving combinations of mods, in the ultra preliminary stages and may well never even
see production, but I’m not really at liberty to discuss the project.
When (if) I get the Okey Dokey to spill the beans I will, and I hope I don’t wad any panties by posting even this much.