Re: JCM 800, 900 or 2000???
As it's been said, the 800 is a mid-gain '80s amp. With its minimal control layout and LOTS of usable tones (virtually you just dial anything, plug in and without whatsoever you sound good, IMO effective simplicity is the main factor of its good fame). It ooozes ROCK but without mods or some booster pedal before it's not capable of metal alone. Anyway, if you crank it like hell, its triple ECC83 tubes will deliver one of the most defined yet nasty tones of the history. It's said it's durable as hell, as my JCM800 user fellows tell the amp box will fall apart sooner than the amp itself. Unlike the 900 and 2000, it produces really good sounds with different power tubes.
The 900 has many differences in the preamp/power amp section, AFAIK some cheaper (yet still effective) components were built in (e.g its tranny), anyway, basically imagine some JCM800 layout with 3 ECC83 tubes + some overdrive pedal in the front, integrated to the PC board. Actually, for bigger dirt the JCM900 uses a diode clipping circuit (except fot the SL-X version that uses an extra tube instead), virtually on the same basis as a distortion pedal that unfortunately can't be bypassed if you feel it won't rock your boat. As a side effect, without CAREFUL settings you can carve patterns on glass with your tone

The controls are pretty interactive so it1s extremely easy to get a thinny and harsh sound. IMO it's 90% on bad settings that made these amps less desirable, plus the fact that IMO these heads sound good only with EL34 tubes. I've heard one that had 6L6 tubes... Terrible! Considering durability my 15ft SL-X test fly and a year of hard tear&wear did not harm the amp, in any mean.
Anyway, THANKS for EVERYONE who flamed these amps so far, without you guys I would be never able to get my superb SL-X for $450. THANKS THANKS THANKS THANKS!!!
The JCM2000 has two versions. The DSL (Dual super Lead, switchable channels that share the same EQ) that is the better brother. Most people use it maxed and that way it's a pretty modern sounding amp, but add strong Marshall favour and some definion and the capability of pumping out the most intensive (yet defined, a Marshall heritage) lows of all Marshall heads, only the ModeFour can top it, but in a flabby way. If you not max the gain, it can produce superb cleans and virtually 90% close dirty sounds of almost any of its precedessors, except for one thing: it has a much quicker response and somehow sharper tone and apart from its scoop switch it makes something with the midrange that some guys hate and some like. I like it. The other version (TSL, Triple Super Lead) has 3 switchable channels. It is one of the most versatile all-tube beasts that these guys produced. You get separate EQ for 2 channels (clean and lead/crunch) and as a consequence a somehow complicated structure. It has plenty of good old and modern tones but AFAIK there are reliability issues, just like with many of the otherwise superb sounding 30th Jubilee 6100 heads.