I owned both the (original) HD800 and HD560S together for about 8 months. I EQed both but it was the HD800 that really needed EQ to be listenable. I also demoed the HD800S recently, and that too would still need EQ for my tastes (it's less bright than the original, but still too bright for me).

In terms of technicalities, there is a lot of exaggeration in this hobby. I'd say the HD560S gives you 90% of the technical performance of the HD800. I actually prefer the soundstage of the HD560S because it is less artificially large (but this is totally personal preference).

Imaging on the HD800(S) is a strong point but it is also very decent on the HD560S. Bass can be improved substantially with EQ on the HD800 but not quite to the level of punch you can get out of the HD560S with EQ.

Comfort is the real issue with the HD560S in my view while a lot of people really enjoy the HD800 for it's comfort (low clamp). I personally, still had comfort issues with the HD800 (I do with most headphones it seems).

My advice, if you really want to try an HD800, is to look for a used unit in good condition. That would be around $600-700 and it's a very good deal, especially if you can EQ. I've seen a lot of people recommend it with tube amps as well. If you don't like it, you can always resell it and not lose much (and there is still decent demand for the HD800).