For whatever reason , the stock HD600 ($27.42) cable costs twice as much as the stock HD600 ($12.32) cable, so I am not sure the HD650 cable is any better quality.
This excerpt is from Stephan Audio Art site.
Performance TIP: The Sennheiser HD600/580 stock headphone cable is MUCH better sonically than the Sennheiser HD650 stock cable. The HD650 stock headphone cable uses considerably more signal polluting dielectric which degrades the audio signal. If you want to improve the overall sound of your HD650's, change the stock headphone cable to the HD600/580 stock headphone cable. That improvement just scratches the surface as far as how much sonic improvement the E-Series cables offer the Sennheiser HD650, HD600 and HD580 headphones.
I found it in this thread for anyone interested to read more of the same: http://www.head-fi.org/t/559642/hd-650-cable-vs-hd-600-580