Pros - Excellent Bass, Amazing Sound Stage, Great Price, Comfortable
Cons - The Highs are a little to amplified, sometimes painful
These are amazing, especially when paired with the EF-5 amplifier. Would recommend to people looking at headphones under $500. The quality and soundstage is unbeatable, works wonderfully with all types of music.
Pros - Almost even tonal balance, just the right amount of bass (and decent depth)
Cons - Annoying frequency reponse peak at ~1KHz
I'll start by saying that I have not owned a great deal of various pricey headphones. These are my best cans so far, but I've listened to a couple other comparable headphones that I'll mention later. Also, I'm not aiming to write a comprehensive review - some others already did a much better job at that than I will ever be able to. What I aim to do is point out the only fatal flaw I've found in these headphones that I didn't see anyone else mention, along with some other impressions.
My background: I listen to a whole lot different styles of music (from folk to IDM to ambient to jazz to blues to rock to pop to rap, just to give an idea). Over the years I've realized that the best sound is monitor sound. The playback device should introduce as little distortion as possible. The first really good headphones I've heard were my friend's Shure SRH-840. Then I bought myself German Maestro GMP 8.35D - awesome headphones widely regarded as fit for studio work. And then I've gotten HE-400.
My system is a Windows 8 laptop -> EMU 0404 USB -> headphones. Additionally, I own the O2 headphone amp and I have tried taking the sound from EMU's linear output and putting it through the O2 instead of using the EMU's built-in headphone amp. Unsurprisingly, I could hear no difference whatsoever. Because, well, 0404 is a solid device. EMU has less power, but it wouldn't mean I would be less deaf if I listened at peak volume - both amps are perfectly capable of producing over-the-top sound pressure levels with HE-400.
Build quality: I have no complaints. One thing I've noticed is a tiny bit of play in the ear pads where they are attached to the driver housings. It's not an issue at all, but something I would reasonably expect $400 phones to be free of.
People complain about the cable connectors, but I've detached and re-attached the cable a couple times and had no problems doing so. The connectors are clearly not convenient, but not in any way problematic. And because they're screwed in you don't have to worry about sudden connection loss.
The painted white logos wear out extremely fast. So would the L/R marks, but those are cleverly placed where you hardly ever touch them so it should be fine.
Comfort: I generally don't have problems with headphones at all, but HE-400 are clearly a bit more comfortable than GMP 8.35D thanks to less clamp force. They're heavier, though. I can feel them on my head, but I've never been uncomfortable yet. The only minor complaint (and I have the same problem with many other phones) is the headband cannot be adjusted to be short enough to fit my head well. It's just a tad too long in its most retracted position, so the earpads touch the upper part of my ear due to the headphones sitting too low. I'm an average height and build person with an average head, so not sure what's wrong there. Perhaps, my ears sit higher on the head than usual? Anyway, GMPs and HD580s have the same problem. I need to almost place the headband on my forehead to compensate for the extra length.
Sound: I won't specify a list of recordings used as some other reviewers do, simply because I've had the phones for years and listened to a whole lot of music from my collection of 29K tracks.
Just one example would be the album "Roadhouses and Automobile" by Chris Jones which is widely regarded as having great recording quality.
At first HE-400 seemed slightly superior in every aspect compared to GMP 8.35D (which, I'll remind you, are very solid phones and that's not just mine opinion). The main difference was slightly wider soundstage, and generally the sound was different in a way I couldn't pinpoint. Then I've listened to Sennheiser HD580 and realized this must be what open headphones sound compared to closed ones. I clearly hear this difference (that I still cannot exactly describe nor do I understand why it even occurs). I cannot claim I hear any specific planar / orthodynamic / isodynamic sound, though - all I hear is decent open headphones.
Bass is just the right amount and very deep. I was afraid these open ortho headphones would not match closed GMPs, but they do!
These phones are slightly more detailed than GMPs as well HD580s, and they seem to have quite flat frequency response across the whole range. But here's the catch, and I only noticed it recently: I was listening to Rob Halford singing "You've Got Another Thing Coming" on the 2005 Japan remastering of the Screaming For Vengeance album, and I've noticed unpleasantly harsh sound. Then I've found some other tracks that displayed the same problem. Long story short, there's a peak somewhere close to 1KHz. I don't have the equipment to pinpoint it, but adjusting the 1KHz slider of a 20-band digital equalizer by approx. -4 dB solves the problem. I would rather use a parametric equalizer, but again - don't have the equipment to find out the right frequency, Q factor and attenuation.
Summary: solid headphones, very versatile, almost monitor sound quality with the right amount of everything except for a narrow frequency response peak at ~1 KHz that will make some tracks sound very harsh and loud (some vocals, harp, trumpet can strike this spot).