Can be tizz or sibilance. Headphones with emphasized S sound regions in sound can have this. D7000 has it, 990 has it, X2 has it, and others may have it. Another term for it is 'hot'.
The 598 and many other Sennheiser headphones tend to subdue these areas, which is why you don't hear it. They're very good at de-emphasizing these areas, but for some, that makes them sound less detailed, or too soft.
You can't have everything.
mad last envy i am very happy i ordered the x2hr
and i aSK MY APOLOGIZES TO YOU AND TO EVERYONE/./
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AND I ASK MY APOLOGIZES//