I'd like to put my 2 cents in here too. While it's true that the ES5 have a very neutral frequency curve, they most certainly are NOT bass-light. Consider this:
I am 38 years old, and have been into car stereo equipment since I was 16 ears old. I use to run 4, 6 even 8 12" subs in my cars. I would dump $15k into my car stereo kit, then run it around North Carolina to all the shows and contests. I could hit 160 db, and more, if I wanted to stand outside my car. Rediculous sound pressure levels, to be sure. But as time went on, I began to pay more attention to crystal clear sound. Still, no matter the genre of music that tickled my fancy, I always needed some thump at the bottom end. Most car stereo nuts know that a sealed box takes more power to hit the same db's, but the bass is more controlled with less roll-off. Add a port and the bass begins to boom at the 65hz mark or so, depending on the length, diameter and positioning of the port, and whether it's a 5th order band-pass enclosure or whatever. Now, my favorite setup requires that the subs are mounted in a non-traditional way, so that the laws surrounding dynamic drivers cease to exist. In this instance, I had 2 subs mounted in an isobarik config, where two subs are mounted facing eachother, only separated by the 2-inch thick plexiglas wall. So that one set of subs was in the box and one set was outside the box. By wiring one speaker of each set out of phase, you get a push-pull effect, that dramatically allows you to "feel" those frequencies under 20Hz.
My point to all that, is that the armatures inside these IEM's (ES5, JH16, JH13, et al..) mimic that isobark "push-pull" setup almost exactly. I am a bass lover, and these ES5 I own are great for bass. But not just trunk rattling bass, where the license plate is vibrating all over hell. Nice, tight, deep, viceral bass that exends all the way down to 8 Hz (provided you turn up the volume).
Still, to this day, I run 2 JL 12's in a custom enclosure between the bucket seats of my Dodge Ram Quad Cab, each pushed by a JL 500/1. Then I have another 500/1 pushing 2 10's in a custom enclosure under the rear seats that are wired for strictly Mid bass thump. I run a 4 channel amp to the Eclipse speakers in front and in rear for fill, and when the wife and kids aren't in the truck, I like to turn it up, ya dig? Sometimes I'm cranking Journey (haters!), other times I have some Russian concertos going. But I like bass, and the ES5's give me all the bass I want. I am also not afraid to EQ (again, haters!) and I can effectively make the ES5 sound how I want, depending on the recording and it's deficiencies.
For any system, headphones, speakers I buy or try, I use a CD that separates the frequencies and gives you the opportunity to hear when a Frequency response drops off. Get yourself the Sheffield A2TB Test Disc. You'll be able to run pink noise, low disortion sweeps, and even listen to individual frequencies from 10 Hz up to 99Hz, which is extremely useful in coroborating Westone's claims of 8Hz. I can at least verify they go down to 10Hz!. You'll be able to check in and out of phase alignment, etc. A great test CD to have at your disposal.
I can't say anything bad about any other IEM's, because I have never tried any. But my need for bass, and what the ES5's provide, do not leave me wishing I could have tried the JH16 instead. I am quite content with the ES5's (and my big copper cables Tronz