My first amp head (and tube amp, for that matter) was an MK120. It's basically a JCM800 with some quirks. The stock tubes are 6N2Ps, which aren't directly replaceable with 12AX7s, and requires a modification to take them. It also uses a 5881/6L6-based power section, and the "Presence" knob isn't the typical Marshall-style one using a negative feedback loop. There's a fuckton of low end (especially noticeable with the cab I was using it with...a G-Flex 2x12).
It's been several years since I've played through one, though. I know that they have a bit of a cult following for doomy type stuff. I had mine modded for more gain, less low end and less extreme treble (as well as 12AX7s). Overall, I remember it being a good-sounding amp, but one that definitely wants to be loud.