Wonder why so many people tell you that IEM are very reveling and full of detail, yet most IEM don't seem to go pass 16kHz when you look at their frequency response? Here is the reason:
Although human hearing range are listed at 20 Hz to 20000 Hz, most people can't actually (or at least have difficulty) hear sound above 16kHz, especially once you pass your twenty. Don't worry about losing treble/detail over 16kHz, as human tend to interpret sound b/w 5kHz to 16kHz as 'high' and most detail are actually on the lower region.
A headphone that is capable of producing sound over 16kHz, 20kHz or even 30kHz doesn't help if you can't hear it, unless you are planning to share your 'phone with your dog. So why headphone manufacturers like to highlight the frequency response of their 'phone when they know you can't hear it? Cause general consumer don't know the fact and often believe 'more is better' with out realizing it is just a marketing trick.
Some of you might have read from else where that human, although can't hear sound beyond 20KHz, can still feel the present of ultrasound (and the harmonic distortion caused by ultrasound) and it does have a positive effect on SQ - This is true. However, giving that almost all CD, lossy and lossless music produced these days have a cap on 20KHz in recording / encoding, plus most sources only equip with a 16bits DAC that ain't capable of rendering sound beyond 20KHz efficiently, we can safely conclude that most music we listen these days doesn't come with any meaningful sound beyond 20KHz that can be reproduced by your headphone or IEM. If you want to listen to a recording that does include sound beyond 20KHz, you'll have to go back to analog record (LP) or get SACD or DVD-A, but mind you, you'll still need equipments that capable of playing those sound back.
For a better understanding of which IEM is more detail, the best way is to read its review (or better yet, an audition). Also, Headroom has done a great job in measuring headphone freq. response, so you might want to
have a look. Note that, although freq. resp. does tell us a bit about how headphone (including IEM)
might sound, it doesn't tell us how good it will sound. When it comes to picking up a pair of headphone, your ears are much more trust worthy than your eyes.
If you are interesting in learning more about frequency response,
here is an article that might provide you with more information.