For a metal tone, I think of one of two things. Either a thin, shreaking, screaming guitar tone, or a really bass heavy, drop D kind of tone.
Here are a couple suggestions for neither, really:
Get a small amp or hook up a tube amp to a small cab. Most "metal" amps are too loud, and a lot of metal players use full stacks, so they can't get all the speaker distortion that they could if they used less or smaller speakers. Speaker distortion can be a great asset in providing tone. To hurry it along, stick a pencil through the speaker cone halfway between the center and the perimeter, equally spaced 2 to 4 times per speaker. I guarantee that will give you a lot of distortion, just make sure you never want to play clean again.
Stay away from the processed tones. They're not unique, and they always come out sounding pretty bad.
Try 10" speakers rather than 12". You can still get a good amount of bass through 10's, and I tend to like the breakup better on 10's.
You can always be unique in metal by being "retro," instead of metal, throwback to a classic rock type guitar tone. Some of those tones are the very best ever recorded.
Most importantly, experiment! Break the rules, whenever possible. One time when I had a band in my studio, we needed a really killer lead tone. I took the guitarist's Marshall JCM TSL 2000 and put that into my Mesa Boogie Single Rectifier. It was pretty nasty, I can tell you that much.
-MD