The following are the biased words of a bloodless retailer, who will profit at your expense should you give him a whit's chance.
I began this exercise with the best of intentions. 'I will delineate the attributes of the RE-400, on everything from a portable unit, to the unlikely pairing with reference gear...after all, it's nice to see if gear can delineate the difference between a dual chipped layered dac or a computer card.' Somewhere I got side-tracked. Don't get me wrong, I did listen to these little guys on all kinds of gear, but I wasn't very clinical about it. Maybe it's that there's been a blizzard here and I was due for a good listen. I somehow strayed onto an old Dinah Washington/Clifford Brown session. Richie Powell just played his ass off. He was such a bopper like his brother, then could turn around and be a George Inness landcape for a rich reedy voice like Dinah's to interact with. (Junior Mance also played beautifully with Dinah and Brownie on another side) You got the message of her voice with the RE-400...all of it. You could also hear certain nuance, like her excellent use of the mic, and texture. That experience made me want to listen to other highly textured vocals. So I hit Andy Bey, Donny Fagen, Jill Scott, Little Mary Callas, Lorraine Hunt Lieberson, Carmen McRae, Irene Kral, Meredith D'Ambrosio, Ray Charles, Abbey Lincoln etc.
I listened for about 5 hours with breaks when switching gear. At hour 3, it was time for a scotch. (Michel Couvreur for those of you who want to try something superb in a Belgian/French/Scotch effort*) With the iFi gear I enjoyed using the minor XBass setting, which iFi recommends for open back headphones like Sennheisers. The RE-400's don't lack bass, I just find with many detailed headphones, that the smallest mid-bass boost can bring some warmth to the table. Like all good cooking, it's how you season your roux.
Next came some Joscho Stephan. The RE-400 did a fine job of delineating the 3 guitars and bass being played at blistering speeds, and got the tone that Stephan favors in the bargain. With him, you really hear the kind of strings he plays. Here's a youtube video if you've never heard him. A bit of a technical monster.
The RE-400's did tell me the difference each time I switched gear. They are accurate transducers that don't do anything wrong. It's rare for me to hear something at this price point that is as accurate without grating on my nerves. The engineering choices were excellent. When I got to my $15k reference headphone amp and dac, did I feel silly? Well, yes, slightly. Still the main thing was that they didn't sound exposed or harsh. They just sounded better, though there were elements that were simply omitted, that you get with the top gear.
They don't image like an HD-800, an HE-6, or a good pair of speakers. They also don't do the spacial things that some CIEM's can do-deep bass and space. What they do well the do very well. Possibly the most poignant thing they did for me was give me a 5 hour listening session. Not all gear can make that happen, and certainly not a great deal in the $100 arena. One last item would be their nature. These are HiFiMAN through and through. If you want bass thumpin/corrupting Beats, keep walking. You'll get real bass here-that is, you'll hear the note and it's texture, for instance listening to Ray Brown or Marc Johnson or John Clayton, you could tell how they were mic'd, if you're into that kind of thing. (pick-up or mic) Then again, if you use XBass' max setting on the iFi iCAN, you can get some of that boom thing, but that's not what these fine transducers are about. These give you real delineation for the cost of some audio fuses. I really like them. Now, I find myself wholly interested in receiving the first batch of RE-600's.
The RE-400 was easy to seal, and held it's seal well once in my noggin. The test equipment is my Macbook Pro, (lossless files using various software implementations) directly from the computer headphone out into the RE-400, then from the RE-400 into the iFi iCAN, then from the computer to the iDAC, then from the computer to the iDAC to the iCAN, then I used an Music Fidelity X-CAN, then the computer to the Metrum HEX to a prototype reference headphone amp. On the vinyl side, there was a variety. I won't go through them all is it's not really topical.