I've found that most players and bands that Use Marshall amps and are doing heavy rock or metal are using signal processors for their sound. Marshalls are good amps but if your'e doing "real" heavy stuff like Slayer and the like, I think you'll find that purchasing a digital signal processor will get you where you need to be as far as sound goes. Just don't skimp too much on the price and you'll find one.

Not for nothing and off topic but I think Reign in Blood was one of the top 3 Metal albums of all time.