I tried the HD660S and was really impressed with the pairing, if I didn't require a closed headphone I would have gone with those. Also I think they come with a 4.4mm cable(?).
The Sennheiser 660 is actually made for DAPS and is not 300 Ohm impedance like HD 600/650. It even comes with a cord that has a 4.4 balanced connector for the ZX300. Go to the Headphone section and read all about it.
Thanks, I'll keep my eyes open for a deal on the 660 then.