Marshall JCM900...

Put a Tube Screamer in front, or if you're after a Trey Azagthoth sound you could try getting a ProCo Rat pedal and put that in front. I've heard some good metal tones come out of this amp. Pretty sure it also depends on what guitar you're using and pickups it has. What guitar and model JCM900 do you have?
 
Eh, it's not mine. It's my new guitarists amp.
He's using an Ibanez with shit pickups. (gonna put in an EMG81 I think)
and I think it needs new tubes.
Not sure on the model number, I'll find out tonight.
What does a tube screamer do?
 
A Tube Screamer is an overdrive pedal that has a unique way of "exciting" preamp tubes... or something. Not really sure.

My personal favorite method would be to turn the presence and EQ knobs to 10 and turn the gain as high as you can without losing clarity.

But yeah, it should be able to sound totally metal on its own. If not, the amp isn't running properly.
 
The tube screamer actually pushes the gain signal on the amp and gives it more gain stages, slightly more saturation and tightens up flabby bottom end. What most people do is possibly put gain slightly more than 0, Level at 9 or 10 and tone at 5 or 6. That's enough for a nice clean boost and to tighten the tone up.

The JCM900 should sound metal enough on it's own. It's supposed to have more gain than the 800, especially the master volume models. Just make sure he's not scooping his mids, perhaps change tubes. They make a HUGE difference to sound.