It's been my experience through multiple headphones that a more neutral or even emphasized upper midrange will make the headphones sound more up-front and in my head.  HE-400's upper-mid coloration helps make it seem very out of your head and layered at times.

Commenting about accuracy of a soundstage though, that's slippery terrain.

There's definitely much more to soundstage than the macro-scale tonal balance; micro-nuances and distant cues / room reverbs in recordings help define the realistic portrayal of the staging, if they are properly delivered by the transducers. HE400 overemphasizes those nuances/cues in the upper treble region, and it's so overwhelming that its soundstage presentation sounds off to me.

Yes I do realize it's a slippery terrain, but I try my best