Well, finally, I don't find the hd800 too bright.
I was trying first to tame the treble by large amount 6db, then 3 db , then only 1.5 db,
but it was just too much, often making the sound less interesting.
So here is (hopefully) my last eq , I tame by less than 0.8 db:
0.8 db might seem an insignificant amount, but I just find the treble is a bit more forward than remaining frequency range,
which I find this a bit distracting, while listening especially to ambient stuff.