I had a GS1000, tried a PS1000 and an RS1i (not really sure about the latter's name) and didn't like them.
When I listen to a song in the above mentioned genres with the HD650 I like it better. Agreed, not a closed can either.
I agree, it's not genre specific like the AT models.
For me it's superb with all genres but the older-harder-rock genres, mostly not that well recorded.
Try something like Jimi, The Doors, Black Sabbath, Uriah Heep, Deep Purple, Iron Maiden just to name a few.
Metallica fares better, probably because their albums are better recorder and the music is fast.
I remember listening to this kind of music on the L3000 and that was headbangers heaven.
Play some tracks of those groups and switch between your TH-900 and HD600 and you'll see what I mean.