I owned the Clear and auditioned the HD800S from TT2/MScaler.I checked out the few HD800S reviews. They are all complimentary.
I did think about the HD800S a while ago. Knowing Sennheiser's reputation for excellence, I had no reason not to consider the HD800S. (I heard that they had addressed the brightness of the HD800.) Then I saw a flurry of remarks calling it thin sounding, especially in comparison to other headphones like the Focal Clear. They say the Clear is balanced and no mention of thin tone.
None of these headphones are for me personally, as I can't live without that perfect planar bass. No dynamic driver will be able to deliver that final bit of bass extension and authority.
That said, out of the Clear and HD800S I actually preferred the HD800S which was a pleasant surprise to me. The Clear is quite forward, occasionally aggressive.
You can read a lot of negative comments on the HD800 (less on the 800S) but they are actually quite good. Impressively spacious and they sound what I would define as 'classic Hi-Fi' sound. Quality, detailed but not much of a character apart from being spacious.
Both the Clear and the HD800S treble can be a bit bright, but on the Clear it is more bothering to my ears due to the more forward presentation.
IMO a good planar beats both of them, at least to my ears, but if I had to pick one between the Clear and HD800S that would be the Sennheiser. (Personal opinion of course.)