Grados for metal is bit two sided. They are excellent for chord thrashing riff oriented metal like Thrash, Death and harder forms of Power/Speed Metal and Heavy Metal, but very lacking in epic or flowery/symphonic Power Metal, Black Metal, Doom/Funeral Doom and or any metal that relies on wall of sound layering, ambience and/or symphonics and soundstage.
So, really good metal combination would be Grado RS1/RS2 together with Sennheiser HD580/HD600 or perhaps even HD650. YES, I'm actually recommending something from Sennheiser, since its actually quite excellent for some forms of Metal as I found out with HD580. Beyerdynamic DT880 is also good alternative if one wants more clinical sound and clarity, but I think Sennheisers euphoricality is more useful more often. This combination can cover about every style of metal I can think about.
DT990 is good all-around metal can, but far from perfect due to searing treble spike and midrange problems (which is bad for metal).
*edit* Also remember, that source and amplification plays a huge role if something sears your ears with highs or not.