Its tuff for me to chose one also so I'm glad I don't have to having both and they do compliment each other great. The reason I think the HE-400 is a bit better technically over all is because I think the portray a bit more micro detail and the bass is so clean and nice sounding but the HD600 is also an awesome HP and I love the fact and can put them on press play and listen for hours no problem. Both are very comfortable to me and I have no problem wearing them for 4 or 5 hours straight. Like I've said I'm so glad a made the jump to HP's over IEM's but I wish I did it sooner because they sound just better imo and the open feel of the soundstage is a big part of that.
Agreed. A deciding factor for many who are stuck between the HD600 & HE-400 is where their midrange presentation preferences lay. I typically can't get on with phones that feature a recessed/v-shaped FQ curve. Regardless of genre, the most crucial part of music (the midrange) is set 'behind' the bass and treble. The midrange is where most instruments, sounds, and voices originate. While the HE-400's tuning isn't explicitly v-shaped, the midrange is slightly recessed. What's unique is how linear the HE-400's bass is slightly boosted, but it's also very linear.. the upper mids through treble are mostly tuned down (lending itself to that 'dark' sound') while the upper treble is boosted. So the HE-400 isn't expressly v-shaped, but it's presentation harkens back to that shape in some ways. That being said, the HE-400's midrange has excellent clarity and resolving ability.. so despite being recessed, it's very clear and easy to make out minute details.
The HD600's midrange presentation is almost the opposite of this: its slightly enhanced, midbass-centered bass tuning and smooth, rolled off treble gives the midrange the feeling that it's positioned slightly forward in relation to the bass & treble. The HD600's midrange is truly magical in it's ability to sound smooth, transparent, airy and liquid.. all at the same time. I'm still not sure which phone resolves better.. many would say the HE-400 is the winner due to it's orthodynamic driver that has a very effortless sound.. but IME, I find mid-fi/hi-fi orthos to compare quite favorably to dynamics they directly compete with (like the HD600). This goes back to how you're driving the phones. I thought the HD600 had hit a ceiling with some of the very nice amps I had tried.. then I listened to the HD600 on a few rigs that were even higher up the 'proverbial' food chain and I was shocked by how well they scaled. At first glance, it might not seem ideal to pair a $400 phone with a $1000-$1500 amp/upstream rig.. but the HD600 is no ordinary $400 headphone! It certainly has limitations.. but I still think it's underestimated due to it being a 'classic' phone.