In my experience (and this is no offense to guitar players) what sounds good to them live is rarely appropriate for recording....a lot of that probably has to do with room ambience and personal taste as well. Do you have a way of isolating the amp so you can judge the sound from your studio monitors alone? That's where I would start. And mic placement (at least for me) has never been as critical in getting a more "metal" sound as much as getting a good guitar/amp sound to begin with. I would try this: record your sound direct not using a mic to begin with. If your sound is horsesh*t for what you're wanting to accomplish, then it's your sound that needs adjusting, not the mic. If it sounds good, obviously it's mic placement. I tend to place a 57 1" to 3" from the grill at a slight angle pointing in. If you want more of a live feel, move the mic farther away. But what do I know.....I'm just a drummer with some recording gear!