quality wise, jcm 800 hands down.
as for sound, it is really personal preference. certain times, i do prefer my mkiii. my jcm 2000 sounds like a compared to my 800 and 900. the 2000 didn't have the balls compared to the older stuff. the green channel was good for crunch, but not enough gain. the red channel had loads of gain, but no balls and doesn't have that signature marshall sound. it was trying to be too versatile and i think it ended up being kinda shitty. the MP1 destroys the jcm 2000 imo. its got better marshall tones and way more versatile just because of the different voicing it has.
i am all about single channel marshalls.
a local vintage guitar store has a marshall major. i didn't see it but they said it is near mint. they want roughly 2000 bucks canadian for it. they are going to redo some caps and do a full service before selling it.
what you guys think? i have no use for it, but sounds like a cool piece to have.