That's really amazing... given... that their drivers/elements are identical. See:
Perhaps that's the effect of the plastic grills vs the metal grills of the HD600s - which is why I got the HD600 grills. My modified HD580s sound identical to my former HD600s.
Yeah, the whole sharing of the same driver realization sealed the deal for me. I bought HD600's grill, did the mod, and that's that. I did add an HD650 headband for durability. But the way the drivers are held by the headband, the better material can't make a difference sonically. There's no way any tiny, inaudible vibrations transmitted by the headband would find their way past that wooly ear muff and affect the sound being pumped directly at your ear by the drivers.