Forget versatility. Not hi gain by today's standards. No metal tone without a boost.
Think so? My JCM800 50 Watter is one of the most vesatile amps I've ever owned. I'd never sell that one! Does everything and in between from Bell-like Clean tones, AC/DC Crunch to Brutal Death Metal (with HotMod or Pedal Boost). They have a great and very usable low mid range!
Those old 800 are the most in-your-face amps I ever played as well. That will bring bad playing technique into the foreground though, so for players with a not so good technique it's better to let them play over a 5150, XXX or the like....
I know pretty well what Brutal Death Metal is (I've been listening to it since the very early 90s), that's why I wrote you'd need a Soldano Hot Mod or a Pedal for that. But those old Marshalls react very well to all sort of Pedals...Gorefest - False is a prime example of a very cool (slightly undergained though) modded/boosted Marshall 800 tone....
I don't think the JCM800 is loose in any way. A Mesa is loose in my book, Marshall are pretty focused, but whatever, different tastes obviously...
But you are right, without Pedal (even I think a standard TS is enough to boost it into High Gain Territory) or mod it won't do Death Metal, or anything Metal at all. But Marshalls just love to be abused and keep their character very well with a Pedal in front.
Heaviest thing I've ever heard, JCM 800 + HM-2 = Morbid Tales.

Do you really think that's a JCM800?![]()
Anyone ever heard of that one trash band called "Slayer"?
Their tones have a lot of gain, even before they switched to the custom KK 2203 model. They didn't play modded JCM's either, they just boosted them with an MXR EQ pedal. With a boost, these amps can get brutal. I remember Suicide Silence also endorsing how brutal JCM800's are.
That's what Tom Warrior told me it was when I asked him, a stock JCM800 boosted with a tube screamer into a Marshall cab with v30s.
If you know different, then please enlighten me.