Ok, some very initial impressions....
The phones fit nicely on the head, not too snug. You won't want to be moving around too much with these on if it is warm, pads are warmish, that is, they heat up the ear area for me.
My first thoughts are that this is a "nice" headphone which means to me that it is fun and warm, the basic Sennheiser sound IME. They are more detailed and soundstage is larger than previous HD models, but the general flavor is still around.
Consider that I am putting them next to the R10s, HE90, SR-Omega in my mind so they have a lot to live up to. They do not have the same spot on tonality or soudstage as the R10s, not yet. Maybe they will get closer when balanced. And I definitely would like to try some different driver tubes with these, I think that will make a difference.
The bass for me is very nice, tight and not overboard, like the L3000 can be on my amp. The amount is just right for me, but I am definitely not a basshead anymore, although I thought I was at some point. When I started listening to the R10s and HE90, that changed. I like a balanced presentation, which the HD800 has. It is not as extended on the top end (as my other phones) but still gives a lot of detail/nice presentation.
Anyway, I can see myself keeping these and listening when I just want to relax and listen to music in a "fun" manner. They do provide enjoyment. I found myself smiling in the usual places when listening to my favorites.
More to follow.....
(Vai, chill, i registered and am waiting for the reply email. Not here yet. Then I will post the graph. We do have lives beyond headphones

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