HiFiMAN needs no introduction around here, so I'll simply share my opinion briefly. In my mind, not only are they one of the premier manufacturers of headphones and related gear, but - and this is possibly more important - they deserve a lot of credit for helping advance the headphone industry in general. Of course, they aren't alone in this category. Plenty of others have done amazing things over the last few years. But I can't think of anyone who even approaches the broad portfolio of HiFiMAN. They have gear competing for best-in-class in a variety of categories - their HE series of planar magnetic headphones, their RE series of IEMs, their EF series desktop amps, their HM series of portable players... all that remains is a dedicated DAC or two to completely cover the full range of HeadFi hear. I've reviewed the HE-400 headphones and loved them. I also use the HE-500 and those are great too. But this is my first time actually having a HiFiMAN IEM of my own. I've heard some of the older models like the classic RE-0 and RE-ZERO, as well as the more recent RE-262, but never got to know them well enough to really form a solid opinion other than "they seem good". HiFiMAN contacted me and asked if I'd like to review their new RE-400 Waterline IEM ($99). Since I write for InnerFidelity as well and have plenty of reviews on deck, I wasn't sure if I would have time for this one. But I decided to work it in and boy am I glad I did. DESIGN Like all the prior RE models, RE-400 uses a dynamic driver. In this case HiFiMAN uses an advanced 8.5mm unit with a titanium driver. Impedance is rated at 32 ohm, sensitivity 102dB/1mW, max power 30mW. So don't try running this on HiFiMAN's powerful EF6 amp as it probably won't be ideal.... The cable is made from OFC copper and has a cloth sleeve leading from the 3.5mm, semi-angled plug to the Y split. From there it uses a soft rubber material leading to the earpieces themselves. I speculate this section was left "unsleeved" to minimize microphonics. It feels kind of flimsy but also strong at the same time and very flexible which is a good thing. Having been in the IEM game for a few years now, I'm sure HiFiMAN knows what they are doing with respect to making sure cables are appropriately durable. Bonus points are awarded for the slider which appears to match the silver shells of the IEMs. The IEM body itself is made from a metallic substance (aluminum?) and feels durable yet lightweight. There's a very small port on the bottom of each earpiece which is fairly common for dynamic based designs. Isolation doesn't seem to suffer - while these are no Etymotic ER4s, they do have moderate isolation as could be expected from a design like this that seems designed for medium insertion. I can get them to go in rather deep if forced but the sound gets worse - I don't think they were intended to be used in that way. Looking at the pictures, or even seeing them in person, these don't seem like tiny IEMs. The Ultimate Ears UE700 or the Jays Q-jays are the two smallest models I can think of, and the RE-400 doesn't strike me as being in the same category with regards to size. Even so, it's definitely on the small side, and real life shows them to be smaller than I had expected. Even with medium insertion the shell nearly disappears into my ear and is hardly visible from the outside. This makes the RE-400 one of the few IEMs to be suitable for listening while laying down. The RE-400 comes with a rather nice assortment of tips - think quality over quantity. I got good results with the typical medium sized single flange tip. But I got even better results from the medium biflange tips. I don't normally get along very well when it comes to dual or triple flange tips so I was surprised at this. These tips have each "layer" closer together than most biflages, and the insertion is barely deeper than the basic single flange tip, so perhaps that's why it works so well for me. GEAR I listened to the RE-400 on a variety of equipment including: Sansa Clip+ Meizu MX 4-Core iHiFi 960 HiFi E.T. MA9 I also used my big home setup based around an Auraliti PK90 player and a Resonessence Labs Invicta DAC/amp. The RE-400 hits the "sweet spot" of usability - it doesn't sound terrible on a basic player, but does scale reasonably well as your source and amplification improves. I appreciate this as some of my IEMs sound miserable on a basic DAP, and others top off too quickly. The RE-400 seems to have something for everyone. LISTENING The first thing that struck me about the RE-400 was the clarity. It's got a very clean, accurate presentation which defies any attempt to stereotype IEMs based on a "dynamic driver sound". If I believed the stereotypes, I would probably think this was an armature design but that just isn't the case. If you think all dynamic IEMs have thumping bass and somewhat slow or muffled highs, you obviously haven't heard enough of them to know better (and specifically, you haven't heard anything in the RE-series). The overall sound falls slightly onto the analytical side. It doesn't go as far as something like an Etymotic, but it does tend towards neutrality and precision rather than relaxed fun. Bass doesn't thunder but what's there is tight and accurate. I find that most of the time it is plenty satisfying for me - any type of rock or jazz, or practically any music using real drums, sounds very nice. Only when trying some hip-hop and electronic music where exaggerated bass is the focus, does the RE-400 seem occasionally lacking. What's there is great - there just sometimes isn't enough of it to rattle your ears in the way you might want. Again, this is a tuning choice rather than a deficiency, and the RE-400 joins the Sennheiser HD800, AKG K701, and many other models with deliberately non-bass-heavy sound signatures. But I don't want to make it sound too bass light - the RE-400 is plenty satisfying for me in all but a few cases. Mids on the RE-400 are really the star of the whole show. Open, clear, and very transparent, they carry the listener further into the music than any $99 headphone has a right to. Vocals sound clear and lifelike. Snare drums have appropriate snap. Everything sounds "real" for lack of a better word. It's quite intoxicating, and forces me to cast an accusatory glace at some of my other (more expensive) IEMs like the $399 Westone AC2. The AC2 has some great bass but could definitely learn a thing or two from the RE-400 in terms of midrange transparency. Because of this excellent midrange, the RE-400 works well with "audiophile" grade recordings. I had a great time listening to Marta Gomez - Cantos De Agua Dulce, Livingston Taylor - Ink, Oscar Peterson Trio - Night Train, and a boatload of classical works from a variety of labels. I even tried some DSD tracks through my Resonessence Labs Invicta and they sounded stunning. I can't think of a better $99 headphone or IEM with which to listen to great recordings. The top end of the RE-400 is extended and clear, but not overly sparkly. For some people this will be very welcome - hot treble is something that really bothers certain people, and can kill an otherwise great listening experience. The RE-400 is nicely balanced - fairly clean and extended without being harsh or overly emphasized. The downside is that some people might find it a little boring. I personally don't hear it this way but I can see how someone else might. If your preference is geared towards the super-exciting, you may find the RE-400 a tad bland. The good part about the highs being the way they are - this is not a completely unforgiving IEM. Sure, it sounds better with quality equipment and recordings. But it rarely becomes completely unlistenable. That's not the case with some of my high-end custom IEMs which practically demand a good source. I recently listened to a flagship custom (which shall remain nameless) on a Sansa Fuze - playing Mastodon's Crack The Skye, I could only handle a few tracks before I had to turn it off. It just sounded like a blurry, harsh mess. I'm very familiar with that album on my home setup and while it isn't exactly a Steve Hoffman masterpiece, it normally sounds far less strained. So I switched to the RE-400 and bingo! I could actually enjoy the music once more. There's a headline in there somewhere: "Extra! Extra! HiFiMAN RE-400 superior to $1,000+ 6 driver CIEM! Read all about it!" This is deceiving of course, because switch to a better source and that flagship custom completely dominates the RE-400 - as it should for the price. But what I'm getting at is this: the RE-400 is not overly analytical and actually does a great job with average quality sources. CONCLUSION I'll keep this fairly simple - the RE-400 is not a massive deviation from the earlier RE-series models. I'd call it more of an evolution than a revolution. It hasn't turned into a bass monster, nor has it gone completely ER-4S all over the place. Still, in some ways it does seem to have developed a more "universal" sound. I suspect the even response and clean sound will work for more people, though a few who previously loved the RE-0 might find RE-400 too homogenized for their taste. As far as I'm concerned, the RE-400 is about as good as it gets for $99, and then some. That includes IEMs and full sized headphones too. I can think of nothing else in the price range that has such smooth, transparent sound, and still remains engaging. Add in the quality build, excellent fit, and well-proven history of customer support, and it seems like an easy recommendation. HiFiMAN does it again!