Hard rock and metal. They don't quite have the speed to keep up. They do have good detail and do a reasonable job, but other headphones do it better.
These headphones don't have the massive-car-sub sound, so if you're looking for that level of bass, well, get an EQ, or get bassier headphones. Otherwise they do fine with most genres. Their best is jazz and classical, but they also do quite well with more downtempo and ambient electronica, lighter rock, vocals, and generally things that emphasize tone and inner detail over dynamics, impact, and speed. They are, however, good all-rounders, and even at their worst they get by pretty well.
They do need amping, though less so than the HD650. The HD650 sounds wretched unamped (imo), the HD600 does not.