I got the HD800s a few weeks ago and listened to them for a few hours and was very disappointed. I had two weeks to return them (or so I thought) and I only listened to them from time to time to see if I liked them better but I mainly listened to my HE-500s.
After two weeks I contacted the seller and he told me I couldn't return them. I had completely misread the return policy on their website.
However, after he told me I couldn't return them I started listening to them more and more, and, surely, I started to like them more and more.
I went away for a week to visit my parents in another country and could only bring my Fiio E10 to drive them. It was far from perfect, but it made me fall in love with these headphones.
I got back today and got to listen to them through my Lyr again, magical! I put my HE-500s on, and to be honest, I much prefer the HD800s now. I doubt my HE-500 will get much head time at all.
It's very hard for me to describe the differences since English is my third language and my audio-related vocabulary is very limited but the HD800 is just so much clearer. The detail and the soundstage are simply spectacular. I just feel so much closer to the singer with these on. Voices aren't so distant anymore.
Also, I mentioned that I thought the HD800 was lacking bass the other day. Now I think that all the headphones/IEMs I've tried have way too much bass. One exception though: for techno and music like that, the HD800s aren't ideal, but I don't listen to that stuff too much anyways so I don't mind.
Anyways, I'm glad I couldn't return these headphones.