Well, the link didn't work for me, but judging by the fact it's a 70's model Marshall, it's probably a JMP 50 watt or 100 watt head.

As far as versatility goes, they're generally known to do one thing, but that's not to say you couldn't use it for other stuff as well. Is it a master volume model? If not, then good luck getting a decent metal tone out of it without turning it up to ear-bleeding loud volume levels. If it is a master volume model, a good overdrive pedal and a set of hot pickups should be able to get you, at the least, a good 80's thrash metal sound. The trick with these amps however, is to drive the power amp hard, and the only way to really do this is crank it. But that's when they sound amazing. You might want to consider a 50-watt head for this reason.

I noticed you said you had an uber metal pedal. Hmm, I still think you wouldn't be using it much if you were able to coax a natural metal tone out of the amp Blues and rock will be much easier to get good tones for out of the amp though.
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