There are quite a bit of discussions online regarding 660S and 700 drivers. Some say 660S is based on 700, but that doesn't mean it is the same driver. Others made measurements and say they have similar impedance and phase response, but then they don't sound alike at all.

https://www.stereophile.com/content/sennheiser-hd-660-s-over-ear-open-headphones

I guess at end of the day, it is how they sound that matters, regardless of their driver design. Let us know your verdict when you have thoroughly compared them.