Lots of things in headphone reviews are subjective, so to give you a sense of my reference points and tastes, I have been into mid-fi and hi-fi audio for about 15 years now. I would call myself a "Basshead Audiophile", as the main types of music I listen to are EDM, Hip-Hop, R&B, Pop/Top40 - all genres that require a decent amount of bass. I occasionally listen to classic rock and alternative. At the same time, I appreciate the detail, clarity and presentation that comes with a good hi-fi set of headphones. My current equipment... Every day use: Sennheiser Momentum True Wireless 3, Apple AirPods Pro, Apple AirPods Max Detailed listening: Ultimate Ears UE11 Pro Custom IEM, Sennheiser IE900, Focal Clear, Audeze LCD-X, Final Audio D8000 Pro, Denon AH-D9200, Fostex TH900Mk2, Fostex HP-A4 (Desktop DAC/Amp), Dragonfly Cobalt (Mobile DAC/Amp) ...As you can see, pretty much all of my equipment, whether low-end or high-end, has an either V-Shaped or U-shaped response curve, and that includes the IE900s. My intention is to use these for detailed listening when I travel, as most of my others are just too cumbersome for that. Now let me get into some thoughts... First, just on general comfort - for me, these are the most comfortable in-ear earphones I have ever worn, aside from my customs that were 3D printed for my ear. The length of the IEM fits perfectly with the natural curvature of my ears, and the small foam tips fit perfectly into my ears and get a perfect seal. The IEMs are also extremely light, which makes comfort even better. It is easy to forget these are in your ear, and you can easily wear them for hours (and I have). The wire is also comfortable, but I do have to mention that it is fairly microphonic (you can hear when it brushes against objects). The included shirt clip helps with this, but it is a potential issue. I have not used the silicone tips at all because I love the foam ones so much. I appreciate that Sennheiser included three different wires with different sized tips, though I would've preferred that they include at least one longer cable, in case anyone wants to use these for desktop use (as they sure could be used for it). Also, of course, everyone's ears are different, and I have seen some folks on forums that haven't had as good of luck with the fit - and fit is key for good sound from an IEM. No way to know for sure until you try them. In terms of sound... Sennheiser is known for lots of different approaches to their sound profiles - all of them extremely well-done. On the consumer/mid-fi side, they have the Momentum series, and these have VERY V-shaped response curves with big, big bass. Then you have their hi-fi line, including open-backs like the legendary HD800s, and these have a very rolled-off bass response, with very forwardly-presented, open highs. In my view, the IE900s take elements of both of the examples I mentioned above. The overall balance here is definitely U-shaped, with a noticeable but not overdone emphasis in the sub-bass region and a lift in the highs, especially in the detail frequencies and upper mids. The cleanliness and separation of the sound, along with the way the IE900s deliver lows and highs together, results in a low-end that is smooth, fast and articulate, with sparkly details layered on top. It is truly well-done. It is simply hard to put into words how smooth it is. Even on tracks that have punchy baselines, the IE900s deliver that punch in a way that is relaxed and controlled at all volumes. It is difficult to find an earphone that does bass so well that it is like having a subwoofer WITHOUT it bleeding into the other sounds. The IE900s do this very well. The bass is not boomy or punchy, but it is definitely there, and shines. What it may lack in quantity vs. some other IEMs, it more than makes up for in quality. Now, I have seen in some other reviews here that the bass is better than any headphone, including over-ears. That is simply not true and is an unfair comparison. The small drivers in IEMs (7mm in the IE900) cannot compete with the bass quality of a driver 7x it's size in an over-ear headphone (for example, my TH900s have 50mm drivers). From someone that loves bass, I was not disappointed in any regard with the IE900s. They are outstanding for their product type. The highs here also shine as a standout, which is not surprising given Sennheiser reputation with detail in the highs. What is great about these is they are meant to be for long, comfortable listens. The highs on these are definitely elevated, and it is immediately noticeable. They are not overly bright overall, but I would say that Sennheiser has taken the highs right up to one notch below too much. This, for me, is a good thing. The highs really shine against the elevated sub-bass, and these do a better than decent job at detail retrieval and layering, so that extra treble emphasis brings out the intricacies of the high end - and thankfully, it is never grating or overly harsh, even at volume - it is another piece of that overall smoothness of the sound. The mids are interesting. Given these are a Sennheiser flagship, the mids overall are VERY good, my comments are getting into the nitty gritty nitpicking. The upper mids are slightly elevated as I mentioned when discussing the highs. This gives female vocals an absolutely superb energy and feel in the mix. They tend to have a nice energetic shine on them. Male vocals, only by comparison to the female vocals, are slightly less energetic. I would not say they come off as recessed, they are just not quite as energetic as the female voices. All of the other action in the mids comes off to me as properly balanced, never congested, and able to keep pace with any genre of music without getting any pieces bleeding into each other. What really makes the mids shine is the positioning, so lets get into spatial presentation and performance. What shines about the IE900s spatial presentation is the positioning. The soundstage itself is not particularly large. The sound goes from ear to ear, no further, and definitely retains what is traditional of IEMs, which is having an "in your head" type of soundstage. What IS impressive about the presentation is the HEIGHT of the soundstage and the positioning of the sounds within that stage. I would say the IE900s produce a soundstage height from about the mouth/chin up to the forehead, and that extends ear to ear. The way the IE900s handle each individual sound and detail within this space is superb. The entire sound plane is utilized. Often with headphones, you get a sense that everything is either coming from the left, right or center - not so with these. You can point to and hear exactly where each sound is coming from, and the layering of the sounds is outstanding for an IEM; this gets back to what I mentioned about the mids. It's not the tonal balance of the mids that shines on these, it is the way they are positioned in the mix to be silky smooth, articulate and detailed. So, while the soundstage size on the IE900s isn't particularly impressive in its size (though I would say it is perfectly acceptable for a IEM), it does get an A+ from me in the way it utilizes the soundstage it creates. Overall, very glad I added these to my collection. While they don't match the performance of my over-ear flagships, that should not be expected. For the moments when I want a hi-fi level listening experience on the go, these are my new go-to IEMs, replacing my custom UE 11 Pros. To quickly compare those two, the UE 11 Pros are actually intended for band members and DJs, and while they produce strong sound, that sound is far more flat and analytical in its delivery, which a much more constructed and in your face soundstage. It is immediately evident the difference in tuning the IE900s bring, and they are clearly meant to be a more engaging listening experience. Now enough writing for me and back to listening :) Read more