I listened to Lake Trout (Not Them You), Failure (Fantastic Planet), Rishloo (Feathergun), and Amplifier (Insider) today on my M50Ss.
You may or may not consider these bands to be metal. What I heard was outstanding. The M50 strikes a sweet balance, for me, between refinement, energy, and detail in a closed can. Truly excellent sound for the above types of music. More fun and more energy than the SRH840 -- not by a lot, but by some.
Couple caveats:
1. my amp delivers an output impedance that approaches zero, which I've heard is important for lower impedance cans (damping, freq response) and I think it pays with the M50 -- bass is tight and the sound is refined
2. I listen at a really low volume, and at this level (see equal loudness contour on wikipedia) the M50 is neither harsh nor boomy nor is the midrange recessed
3. I did the coax pad mod (9.5"), which reduces high frequency energy just a bit, increases the sense of space or depth, and makes it considerably more comfortable and non-fatiguing, to the point where instead of wanting to take them off at some point I like to leave them on