I already owned and sold the HE-6 in favour of the HD800 and HD650 before, though that had a lot to do with comfort. Even in terms of sound, the HE-6 weren't leagues better than a well-amped recabled pair of HD650s. I posted a comparison earlier in this thread. As sphinxvc said, nothing blows the 650s out of the water.
I've found that planars do bass like no other. Nothing competes with them in that regard. They also have a very grain-free sound...something that's difficult to find in dynamic cans. Moreover, all the planars I've heard have sounded "larger" than regular headphones. What I mean is it feels like you're listening to large speakers...the sound images are enveloping and visceral, like a huge wall of sound. But, when it comes to layering and depth in soundstage, they fall behind the high-end dynamic offerings, including the HD650.
The HE-500 are a less technically accomplished but more tonally pleasing version of the HE-6, so based on my previous findings, I don't think they'll be a huge improvement over the 650s. I'll see when I get them.
Yeah... as I've posted much earlier... few phones are significantly superior to the "new version" HD650's when driven by a properly matched set up - even including the Stax phones. Perhaps several phones are "a wee bit better," but not significantly better.