I've rarely found any metal to sound good on horns or other dynamic speakers. It is mastered to sound good on typical home stereos. Metal sounds best IMO on lower efficiency but fairly large multiways, but can be OK on low efficiency single drivers, like say the cheap delco fullranges in your camaro... I've listened to enough metal as a teenager for one lifetime already, but that's a different subject.

To sum it up, I think it's more a matter of efficiency and low distortion. Heavily produced rock and metal thrive on a bit more compression and squashing of the peaks even at the playback end; while classical, jass and other well recorded music sound best with higher fidelity. I doesn't surprise me that little fostex FR's in small backhorns sound pretty good with metallica, as they are not that efficient and no doubt squash dynamic peaks.

Greg B