Wow, this thread has blown up since I last posted. I just got my first bona fide desktop amp, the Violectric V200, and I decided to do a quick check to see how the ES5s sound with it and my Lavry DA10 as the source.
I really haven't taken the ES5s out much since I got my UE Reference Monitors; every time I try to do an A-B comparison between the two IEMs, it's immediately apparent to me that the UERMs have more crispness, transparency, and realism. The ES5s just sound noticeably more veiled, muffled, and clouded to me. There is definitely a sweetness that the ES5 possesses, but I just can't get over how much more open and effortless the UERMs sound. In addition, the ES5s are so sensitive that they reveal hissing/humming with high-end amps, and they're too loud with the portable sources I've tried (most dramatically with the CLAS, which I admit is an extraordinarily hot source).
Using a cheap mini-to-stereo adapter, I plugged my ES5 into the V200 and immediately heard a conspicuous hum from the amp--even with no music playing. I then plugged in my UERMs and heard virtually no hum or hiss. I won't go into detail about the sound quality of the ES5 with the V200 (which I felt was clearly below what I heard from the UERM), but the humming I heard with the ES5 was disappointing. The ES5s seem too sensitive for many portable sources and amps, and also for higher-end amps with more power.
I hope I'm not misjudging the ES5, an IEM which I really enjoyed until I got the UERM. My internal comparisons between the ES5 and the JH16 would usually lead to a toss-up, with maybe a slight preference for the ES5. But now I can't really make time for either of those IEMs, because I'm finding the UERM to have significantly better synergy with my equipment (i.e. less hiss and more volume range), and it's also obvious to me that the UERM is providing more clarity. I ordered a Pico Slim a few weeks ago in the hopes that it would give me the low gain that these sensitive IEMs really demand, so I'll see if the Slim will somehow unlock something in the ES5 that my other amps haven't been able to.
Ultimately, though, I don't foresee the ES5 sounding better to me than the UERM in any setup. I'm happy that so many people are enjoying the ES5; it's an awesome IEM and Westone is a very responsive and reliable company that (IME) makes near-perfect fitting customs. But I'm a bit mystified that the UERM hasn't enjoyed a better reception on Head-Fi--is it the low number of drivers? The emphasis in marketing materials on "neutrality", which some may interpret as boring or sterile?
I have yet to hear a custom IEM that fits so many different sources and amps, from the headphone out of a Classic, to desktop amps and DACs, to a studio mixing board (which I heard at the NY meet). If you have a chance to hear the UERM demo at a meet, it's an opportunity that shouldn't be missed.