So if the HD-800's sound bright then perhaps it is due to the source equipment. Or put another way if the source equipment you already have is 'tuned' to other headphones where a bright signal matches with them quite well, then it could very well be too bright for the 800's
Now I'm not saying that the 800's don't accentuate some of the upper frequencies because clearly they do. But if the source electronics also boosts the upper end, well that would explain some of the bright comments.
Just a thought or 2.
IMHO you have hit the nail squarely on the head. Criticisms of the HD800 reflect on bad input IMHO. They are truly revealing & GIGO applies with a vengeance.
So, for me, the DAC is the critical factor. Poor algorithms in the digital filter or poor jitter control and you will hear it in those Sennies. I'm not trying to say all amps sound the same but the amp will just pass on deficiencies spewed out by the DAC & maybe add a couple as well.