The RE-400s' sound is so well-balanced and pure, at first you might not realize how good it is. Its unforced clarity is easy to listen to for hours on end, probably because there's no boosted treble or trickery of any kind; the RE-400s just go about their business sounding accurate and natural. Densely mixed electronica or big jazz bands with a lot of instruments are well served by the RE-400. The remarkable clarity lets you follow each instrument and musician more easily than headphones with boosted bass or overaccentuated treble. The RE-400s also do a mildly better-than-average job of blocking outside sound, which also helps improve clarity.
The bass, midrange and treble frequency ranges are perfectly proportioned, so the RE-400s sound great with most types of music. Bass fanatics may crave more boom and weight, but the bass is all there, and bass definition and detail are exceptionally precise.
Ernst Reijseger's chamber music soundtrack to Werner Herzog's film "Cave of Forgotten Dreams" was recorded in a large church. With the RE-400s, the sound is so good you feel like you're there, and headphones that can do that resolve the tiniest, quiet details, the really subtle stuff that other headphones gloss over. If you've never heard a great pair of in-ear headphones like the RE-400s, you don't know what you're missing.
By contrast, Klipsch's Image S4 in-ear headphones sounded muffled and diffuse next to the RE-400s. Drums and percussion on the S4s were harsh and gritty -- at least that's what I thought after I heard how much more natural they sounded over the RE-400s.
I enjoyed the RE-400s most with acoustic music of all kinds, as the combination of detail and warmth brought out the best in that music.
Etymotic Research's $149 hf5 headphones were slightly ahead of the RE-400s in clarity. Both models present vivid, you-are-there sound. The hf5 is a sophisticated performer, but when I played a few LCD Soundsystem tunes, the RE-400s fleshed out the sound a bit more without sacrificing transparency. Bass was better, too; there was more of it, but bass heads will prefer the $99 Velodyne vPulse in-ears. They totally trounce the RE-400s in bass. Then again, the RE-400s definitely trump the vPulses in overall clarity and refinement.
The HiFiMan RE-400s offer audiophile-pleasing sound quality for a very reasonable price -- just $99. All types of headphones, even very good ones, have different trade-offs, but the RE-400s excel in clarity, beautifully balanced bass-midrange-treble, and noise isolation, and they're quite comfortable. The one group that might not be swayed is buyers seeking heavyweight bass; the RE-400s will likely sound bass-shy to them. The lack of a mic and phone controls might also be a deal-breaker for some shoppers.
Editors' note: Currently, the RE-400s are the only in-ear model offered by HiFiMan. But the company says it will expand the line in the near future with additional, higher-end models.