I compared them in this review: http://www.head-fi.org/products/akg-k712-pro/reviews/9953
Quote from it:
Comparison to the HE-400
When you compare these two headphones, the weird V-shaped frequency response of the HE-400 is very noticeable. The HE-400 has a great bass presentation in my opinion, and the texture on drums is brilliant. On the same time, this combined with the rather small soundstage gives you this very intimate and warm sound that can be a bit too much. Vocals also sound a bit strange on the HE-400 when compared to the K712, this is probably related to the dip between 5-6 KHz. HE-400 also has slightly more emphasis on treble, especially sparkling sounds like cymbals, sometimes this works out great, sometimes less so. HE-400 definitely has a higher fun factor, so this does boil down to taste. I would still say though that the rock solid performance of the K712 makes it better overall. There is slightly more sibilance on the HE-400.
AKG K712: Is more neutral, has a larger soundstage, is an more even performer.
HE-400: More bass impact, slightly more treble, does some material very well while falling short on other, more forgiving with bad recordings
I have actually compared them again after writing that, all impressions stay but vocals sound even weirder on HE-400 when you've gotten really used to K712. In some ways I feel like the Sennheiser HD-650 is the perfect compromise between the two, getting a sweet "fun" sound while fixing that weird midrange.