I'd like to add something about the "accuracy" of these vs. other earphones.
The ER4S are extremely flat in every area of frequency response except the sub bass. But even the sub bass rolloff is gradual and smooth. There are no major bumps or dips in the entire range. This has one major benefit over other IEMs. You can easily make them unbelievably flat with a simply bass boost eq that gently rolls into the 80-100 hz area.
Other IEMs have so many boughs and troughs throughout the range that it is difficult to EQ them to a flat reference level without a graph to go by, and even then it's difficult. Even something with a seemingly simple curve adjustment is still not easy to get "really" flat with EQ without using a graph or tool like accudio. The er4s however is a simple boost smoothly rolled off into the 80-100hz area. When I say rolled off, I mean there shouldn't be much boost in the 80hz area. It should be approaching the normal level at that frequency with no more boosting. As you drop down lower you should gradually increase the bass boost until you're around 5db or so at the 50hz mark roughly.
This is easy, because first of all, a lot of devices with bass boosts have a fairly similar boost region. At least it's not hard to find one with a similar curve with a little effort. Such as the arrow headstage amp. And with a parametric EQ it is a 20 second configuration and you're done. Want more bass? Raise your eq a tad. Less? Lower it a tad. Try that simplistic fix with any other IEM... Not gonna happen with any that I've seen so far. So, while none of them are perfect, the er4s is extremely easy to bring to a very neutral state across the board. And if gnarlsagan is right, the other "slight" (very slight) imperfections in the treble such as 1-3khz or so should be much more reduced using the red? filters. I plan to try these myself and see how they are.
Anyhow, just some thoughts for any potential er4s purchases. :-) If there was anything I would do again in my IEM search it would be to not go the custom tip route. Unfortunately, I just don't think it's easy or consistent enough to get the quality I personally would expect. I would have stuck with the stock tips and/or stock foam or complys. Lastly, I would have tried EQ sooner. I've been anti EQ for a while, not because of any supposed lack of quality, but rather, I prefer a consistent sound across my many audio devices. Doing this with EQ is typically less than optimal. However, again, with the er4s the eq is so simple, it's easy to apply to any device pretty consistently. Once I started using the denon EQ on my ipod, I was amazed at the quality. Then I applied it to my desktop and itunes, etc. Now I'm a happy man. :-P
Edited by luisdent - 6/16/13 at 10:59pm