I got some bad news for you guys,
some one owns an HE 400 and a X1...
I listen to a wide variety of music - vocal ballads, classical pieces, orchestral pieces, EDM, jazz, pop, hip-hop, rock, guitar/piano instrumentals. I also use them for games, movies.
In my opinion, the HE-400 is technically weaker in most aspects compared to the X1.
Let's try to break it down -
The airy sound has no real imaging quality - listening to the HE-400 is like standing in a huge hall with a lot of instruments laid out all around you, which isn't the case especially when you listen to orchestral pieces. I've been to a few live orchestras (obviously not nearly enough :P) and based on my opinion of what natural and realistic imaging is like, it's a lot more to do with depth and the cohesiveness of the whole soundstage. The X1 lays them out like a real orchestra, all in front of you, with a wide and deep image, meaning you're able to pick out which instruments are behind and which are in front, which sections are laid out where. More than that, when it calls for back to front imaging, the X1 does it well as well - try listening to binaural recordings - the X1 does all directions unerringly well.
In terms of mids there is no contest, the HE-400 although does a decent portrayal of vocal sounds, its not natural in reproducing upper register voices and sounds. The X1 gives me way better vocals - I feel like singing together with them and I get goosebumps on certain vocal ballads.
In terms of treble, the X1 is more "laid back", but is more accurate than the HE-400 which is way too hot. Take a look at the goldenears graphs and you'll know what I mean http://en.goldenears.net/17992
The mid to upper registers are almost perfectly laid out to "ideal" FR (tested and used by nearly every audio engineer in the world).
In terms of bass, well this is more polarising. One has planar magnetic bass. Deep, textural, precise. One has dynamic bass, slightly warmer, looser, and "natural". However the clincher comes here - the X1 has more accurate bass imaging. Listen to any pop or jazz songs, and if you have both the HE-400 and X1, do a comparison and try and notice where the drum sounds come from. The HE-400 has the drum sounds coming from all over the place - its hard to pinpoint where the snares, bass and hats are. With the X1 you can precisely note that oh, the bass and snares are from behind the guitarist or vocalist, and the layout is roughly so so and so.
Perhaps with certain music genres the HE-400 shines, such as EDM, where "naturalness" and precise imaging doesn't matter as much. But for a all-in-one headphone, it's really really hard to beat the X1 at its game.
Anyway, just my opinion with some analysis. Like all things audio - sprinkle with salt as necessary.
Yeah seems like a lot of people said that. Unfortunately I don't "only" listen to EDM. But fair enough it's bass is one of a kind, though not really realistic (EDM isn't, but other stuff I listen to needs natural, slightly slower bass).
Seems the X1 is warmer and more natrual :O oh and it doesn't need an amp