Besides Jude's quote also just came across this...a bit more insight to the 58X driver
"The driver of the HD660S is slightly larger in diameter so the drivers are definitely not the same. And NO this is not a HD555 driver either. Yes, the same magnet and plastic frame is used (as in many Sennheiser models) but has a VERY different voicecoil (different impedance) and membrane/cone."
The same site also gives instructions for modding also....download the pdf here.....YMMV
"Those looking for a low cost HD660 replacement can be quite happy. The good folks hoping to find a real HD580/HD600 replacement preferably with a 'fixed' bass response and less 'hot' around 4kHzcan also be very happy."