Are you powering the HE-6 with a Lyr? If so I can see why you'd prefer the HD700. There's no contest with more power feeding the HE-6, IMO.
I just a pair of these on loan today and I'm pretty impressed. I also received loaner sets of the HE-6 and HE-500. I would say that both the HE-6 and HD700s are superior to the HE-500, which is no surprise, given their cost. However, I'm having a hard time deciding whether I prefer the HE-6 or the HD700. It will take time to become familiar with both sound signatures. Unfortunately (or fortunately?) my dealer sent me a Cardas Clear cable for the HE cans, which tends to give them an unfair advantage when comparing against the stock HD700 cable. I'm pretty amazed how how much of an improvement the Clear cable provides.