Pros: Natural Sound, Build Quality, Easy to Drive
The HE400 from Hifiman is a great value. For someone looking to a.) try a planar magnetic headphone or b.) find a pair of cans that will offer excellent, natural sound at a solid price, then this is one you need to consider.
They're built like tanks! Although the amount of visible plastic is scary at first, its very obvious once you pick them up that they're going to take a few knocks before you have to worry. From the headband (very comfortable) to the metal gimbals to the shielded cable to the plush earpads, the HE400 won't make you feel timid in handling them. Also, since it comes up all the time, I've had no problems with the cable connectors. I can't say why or why not, but they don't seem problematic to me.
It also bears mentioning that the housings are very large. Like huge. Largest I've seen. But they don't touch the ear at all, obviously an intentional design to create a room for the ear (or because of the massive planar magnets required).
Now for the sound.
First word that comes to mind: natural. Nothing sounds "enhanced" to these ears.
The bass is solid and reaches quite low without making me feel like I've got a subwoofer on my head. One of the best ratios of quality to quantity I've heard.
The mids are very clear and neither forward nor recessed. Upper mids are a touch more present than their lower counterparts, but neither sounds unnatural. I can hear all vocals, guitars, and midrange instruments very clearly.
The treble is nicely extended but never fatiguing or harsh. I've read a few reviews claiming the 400s were too bright and I'm very glad to say that I disagree. I could see that they wouldn't play too nice with low quality source material, but as they are, the treble is once again, very natural and clear.
The depth of stage definitely bests the width although both are still very good. I assume that the room in which my ears are sitting is a direct contributor to this. Once again a very natural sound, no depth so great that I'm left feeling that I'm 30 rows back, but also not so forward that I feel like I'm squished between the bass and drums.
The separation is good, not great. My Grados best it, but that's not to say that its non-existent. Its just not studio recording quality.
All in all, I'm glad I've gotten my hands on a pair of HE400s. Although they're a little on the heavy side, they are still rather comfortable and have a great sound. Very organic and natural. They certainly sound best after amplification, but they don't beg for it. Just remember if you're looking at planars, some meaty amplification is gonna get the best out of them for you.