I'm no expert by any means but recently (meaning just a couple of weeks ago) got the HE-400's and a pair of HD650's. The soundstage of the 400 I find missing. I still think there is a lot of confusion by what people mean when they say sound stage. I describe it this way, there is width and depth for me and WHERE the vocals seem to originate from.
The 650 the vocals are centered at my forehead and a little in front of me. This is a nice stage and there is lots of texture across instruments depth and width wise in the midrange, which is what I'm partial to. It IS very euphoric, like you are sailing on waves of music.
The HE-400, the vocals seem to originate at the tip of my nose. It feels very different like the vocals are coming our of YOUR head rather than at you. It is a more intimate sounding headphone. There is still a lot of width but not so much depth in the midrange, because they are midrange deficient. What I can hear on the 400's though is beautiful sounding..bass, treble a little peaky but it doesn't bother me. What's strange is that I find I miss details on the 400 in the midrange that I pick up on the 650's YET at the same time the 400's sound more crystal clear with vocal lyrics more easily heard. I still can't figure out how this seems to happen? Also with the 400's there are at moments absolutely stunning realism in vocals. It's not consistent though.
I don't think I can part with either pair right now. Also bought an Asgard 2 with them and am very happy with that as well. Weirdly I'm also happy with my Fiio X3 with both sets of headphones as well.