Hearing is a truly personal thing, we all hear a little differently, so YMMV...
but to me the Sennheiser 6XX sound is a little veiled in the highs. Like there is a thin cotton curtain over the speakers. They are good for long listens, they aren't fatiguing, but my ideal response in both the very low end and the higher highs is more aligned to the Sundara. I think this is because it's a planar but again I'm no audiophile. People seem to love Sennheiser for the mid range and I have no argument with that, I hear that too. As I said, it's no coincidence Senn's lasted for so many people. I think they stomp the competition in quality for price.
I actually prefer the new IE600 from Sennheiser, their in-ears, to the 6xx, so it's not a brand thing, we just all like different tunings - and even from the same company there is variance within a line. The 6XX is not the 600 nor is it the 660S.