Re: Jcm
You'll get different opinions from everyone, but among tone gurus, the basic line of thought is that once Marshall introduced channel switching, it was all down hill from there. I have a 1985 50W JCM800, which is considered one of the holy grails of metal tone, and I really like it. You can't crank the gain, tho, because the added gain stage (Called 'Boost') is actually solid state, and it gets a bit buzzsaw-like. For an old school tone, take a JCM800, crank the gain 1/2 way up, and then put an overdrive pedal in front of it.
I'm sure my opinions will be met with opposing viewpoints, but this has been my experience with the whole 800 v 900 v 2000 issue.