Well, either I accidentally my GR06 or idk.. Initially I didn't notice any sibilance, but now I do. I listen to them from my iPod Touch 4G with the volume bar at around 40%. It's kind of reminding me of my GR07s pre-burn in.
Nonetheless I must say that these are of amazing value. When I first received them and compared against my GR07, like what Mark said, sound like they just cannot be a (a little bit less than a) third of the price of the GR07..