@OP Passlabs X250.8 with an appropriate pre-will work very well on metal. As reflected in many of the comments, you need the ability to play loud with lots of control.
A pair of Parasound JC1s would also be ideal - preferably pre owned. Since they moved to the JC1+ they have gotten very expensive.
Both the Pass and Parasound are biased in to Class A for the first 25 watts so they are pretty refined on acoustic music too.