I play music like Pantera, Machine Head, Trivium (I know they use a Marshall...)

Are they any good?
Sounds like you need to do some looking up, Heafy uses a Mesa, only Corey uses Marshall. Randall sounds fine for what you do though, I dunno if I agree with their recent focus on making tube amps but that's just my opinion.
The G3 amps are nice. They are one of the few Hybrids that sound decent. One problem some people have though is that the volume will cut in and out. (It is not common, but it happens.)
Okay, Dime used Randall Century amps for studio, Warheads live. So, if you like Dime's tone, lean towards randalls.

Machine use the Peavey 5150 exclusively, and that has some amazing power to it, so i'd recommend that (plus you can nab one cheap used).

Trivium is interesting. Early Trivium (their first album, which i forget, and Ascendancy) Matt used a Peavey XXX, which i've played and love. Then, for The Crusade, he used a Marshall DSL head with a Maxon OD808 as a boost. Corey uses a Marshall JPM (or whatever it's called) Preamp, and a DSL head if i remember correctly. Recently, Matt switched to Mesa, then to Krank, then back to Mesa for live amps. For Krank, it was a Krankenstein most likely, and for mesa he used Rectos and the Mark IV.

Out of all those amps (and depending on your budget), the 5150 and XXX sound great, and aren't expensive. If you have the money, get a Mesa Mark IV....they're suppose to be godsent.