Ok, so seeing how old this thread is, my answer is irrelevant, but I though I'd share my impressions nevertheless. Last year I attended the Vienna meet, where I had the chance to audition both of these headphones, and I have to say that out of all headphones, and there was a lot, including Th-900, He-6, T1 and so on and so forth, these were the two that really stood out to me. Oh yes, and the Fidelio X2, but that is black magic for the price and is a totally different story.
So, both of these headphones were powered by an IFI iCan, and neither of them was used in ideal listening environment, the meet being quite busy and filled with noise. First impression of the HD800 was the feel and comfort for me. It was the most incredible feeling to have that beast on my head, and when music started to play, I was introduced to that nice harsh bright sound, wich, for some reason attracts me a lot. Way more than a bassy can for example. Ideal sound signature to say the least. I was impressed by the airiness, level of detail and how it caught my attention. It really made me listen!
Now the grado is a comparison only because they have a slightly similar sound signature. Bright. And to be hones, that's really how I like me music. The first impressions with the grado started with the music playing. It was far from comfortable or a nice feeling, but man, that soundstage! Just incredible! I was in the music! I found myself turning all around and looking for the sources of sounds, it really made me feel I was in the hall where the music was recorded.
As a wrap up of the most that came to be much longer than planned, I have to say that the HD800 is better from a technical standpoint, but oh the Grados made me drop my chin, it was a whole new world! Sennheiser was just a headphone, only much much better in all respect. Grado was nothing capable of description. Unique, albeit specific to music and amplification, but such a joy, and undoubtedly superior in certain cases.
Cheers, and a merry Christmas!
Almos