For those considering the 820, please make sure that you are able to return it if you are going to try it. There is a well known issue with fit and seal. I sold one for a friend of mine, he really could not get it to fit and seal and he gave up in frustration after trying to get a good fit. I had it on hand for a few weeks myself and I also could not get a proper fit and seal, and I tried. With the poor seal the bass was anemic and the sound very thin and clinical.
I am not saying that this will be your experience, clearly many people find it to fit and seal well for them, but make sure if you are buying it used for example, that you understand that there are certainly enough people out there who have not been able to get an acceptable fit and seal. I have owned the HD800S/650/600 and have an HD700 around, and none of those models gave me any fit and seal problems so it isn't my head shape, per say. And my friend who couldn't get a proper seal, his head shape and size was different than mine so please do your research and preferably buy new so that if you are unable to get a good fit, you can return it.