They really do excel at low-volume listening. Moreso than any other headphone in my lineup. I know that's a quality that is traditionally associated with Grados, but the HD-650s are positively smooth at any volume (no doubt because they're tuned among other things for dynamic, full-range orchestral music reproduction - it wouldn't really do if you lost all detail in a quiet passage and had to turn it up only to be deafened by a crescendo!).
I am fairly certain I have the pre-white-material versions. I ... am not convinced, and I guess there will be skeptics, but - I am not convinced that there should be an expected difference between a darker colored but acoustically transparent thin material and a lighter colored but acoustically transparent thin material. It seems too coincidental to me - they've switched to a lighter color and it brightens the sound? I just don't see it. It's so thin, thinner than the finest pop filter I've ever seen in a studio, maybe there is some material quality they like about the new stuff but in terms of sound I sincerely doubt that the stuff that seems to act primarily to keep crud out of the enclosure has an "unveiling" impact on the sound. It's all too convenient, if you'll pardon my repetition. If I painted the whole headphone a lighter color it wouldn't sound brighter; unless the driver or the sort of acoustic honeycomb structure of the mounting/enclosure itself has changed I don't think there's any basis for a sonic difference (and there's plenty reasonable doubt, in my opinion, given that comparisons would mostly be between older, more used headphones and newer, less used ones). And why wouldn't Sennheiser advertise an improvement, were it the case? I just don't grasp it, I'm sorry, no offense to anyone's subjective perception intended at all.
The damping material surrounding the driver/capsule has been changed from black acoustic silk to silver acoustic metal mesh. I have heard both at some length and the newer model has a brighter sound (across the spectrum) with both the 600s and the 650s.
Some prefer one, some the other but the description about the different material is mentioned by Sennheiser. I remember reading it, I just cannot find where at the moment.