I briefly auditioned the HE-400 at a local Head-Fi meet (I think it was on a Peachtree Audio setup?) and didn't think too much of it, but again, briefly.
It is interesting to see so much being talked about the HE-400, mostly good things. Is it because it's an "affordable" planar magnetic headphone? Tyll at Innerfidelity didn't like the HE-400 enough to do a full-on review for it, and the measurements do look kind of wonky, especially the 300 Hz sine wave response, THD+n, and impulse response. I know measurements don't tell the whole story, so it's intriguing to me to see this discussion.