I have a Sundara and a HD600, among other cans.
I'd say that it's not worth it.
Comfort-wise, the Sundara is significantly heavier (or should I say that the HD6-- are particularly lightweight, which is amazing).
Sound-wise, the Sundara is slightly bright. It has better bass extension, but that can be dramatically improved in the HD6-- simply by EQ.
Vocal-wise, the HD600 is better than the Sundara. I don't know about the 660S.
Soundstage-wise, the Sundara has a bigger soundstage, but it's nothing to be amazed about. I like the intimate soundstage of my HD600.
I think the Sundara would be a side-grade from your Sennheiser cans. It has great reviews, but that's not what my ears feel for the moment.
Currently if I had to choose between my Sundara and my HD600, I'd keep my HD600.
What type of sound are you looking for ? Neutral ? Bright ? Warm ? V-shaped ?
What type of comfort are you looking for ?
Do you have access to EQ ?
Is imaging very important to you ?
Those are four major questions IMHO.