I didn't have the HE-500 and the HD 700 at the same time, but I'm listening to the HE-500 right now and it doesn't seem to have quite as much treble energy as the HD 700. They both have very good deep sound stages. The HE-500 I think has a midrange that is more expansive, but the soundstage of the HD 700 is better overall. I eventually upgraded to the HD 800, but after a few weeks I'm finding the HD 800 not as engaging as the HD 700 nor the HE-500.
It's a tough call because I started out really liking the HE-500, but found it limiting after a short time. I really liked the HD 700, but eventually found the treble too hot for certain recordings and I kind of burned out on it. I turned to the HD 800 with the Analixus mod 2.0 (which did help a lot), but the HD 800 just doesn't have the liveliness of the HD 700 and I swear it's actually brighter, though perhaps less fatiguing with the Analixus mod. I think now that they have reduced the cost of the HD 700 to $649 it's an absolute steal. I can't believe it's priced only $50 more than the HD 650.
I now feel that despite all the benefits of the HD 800 ring driver the treble spike it causes is a deal breaker. Too bad I never tried to mod the HD 700, I wonder how that would have turned out?