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What is the difference between the Mesa Boogie 50 caliber and 50 caliber plus? Would the caliber be suited for modern metal (Nevermore, Arch Enemy)?

I have a .50 Caliber combo.

The difference, as far as I know, is that the early model .50 Caliber amps, like I have, are EL-84 power tube based, whereas the .50 Caliber Plus models are 6L6 power tube section designs.

Which is better for metal? I think that's a matter of personal preference and opinion. EL-84's are noted for their great crunch tone when pushed pretty hard. I'd use mine for metal (if that were my main interest, but I'm more of a blues-based type of player). IMHO, some amps that are called quot;metalquot; amps, sound mushy to me. I won't name any names as I don't want to offend anyone for their choice of amps.

Can it be rebiased for EL-34's?

the 50 Cal has an added EQ