I agree with Xenophobe, try an external eq before plunking out the cash for a new cab. A good eq will always help your amp sound better.
If the eq doesn't get the job done, try the oversized version of the Mesa Rectifier 4x12 (the straight one would actually be better for lows than the angled one, by the way) should solve your problems.
It's also loaded with Celestion Vintage 30's, which are the best speakers for metal, in my opinion.
If this cab doesn't fix your problem, you need a new head unit.