First of all, stop killing yourself immerse yourself in everything UM3x because you're stuck waiting for a parcel to arrive. I should know, I was doing it for two weeks until yesterday arvo. Saying that, here's a few things to note, coming from the ER4P myself. (I'm not very experienced, having most of my listening-time spent on bookshelf speakers rather than headgear, so take the comparison for what it's worth)
- It did not disappoint. At all. I love it
- It's not as bright/sparkly as the ER4P, but give it 4dB of boost (and -4dB precut) at/above 10kHz and it will come very very close. I'm running 3dB and the only song I can tell the difference in is the solo at 6:00 in Sidewinder by Avenged Sevenfold (but then there aren't very many passages with I know that well.
- The UM3x are more fiddly to put in and get a good seal with (because the ER4s don't have the need for the memory-wire hooks, given their deep insertion lenth.
- I find the UM3x easier to get a very consistent fit/seal with (ie nozzles the same place in the ear canal every time). With the ER4Ps I had to adjust the left earpiece in/out to get the bass and treble perfectly consistent with the right one. Id be interested to see impressions of my left vs right ear canal, because with the UM3x I have to use the large/long comply tip in the left ear to get a perfect seal easily, whereas I can do it with the medium/long tip in my right ear (more comfortable using a smaller tip)
- The ER4P might be more detailed in some ways (would have to A/B them to be sure), but I find the UM3x doesn't give up anything and that it presents them in such a way that you don't have too look for them. What I mean by that is that while the ER4P showed me details in songs that I never knew were there, they were almost isolated from each other and I was concentrating on them one at a time. It was beautiful, but I felt like it was hard to listen to a small detail without zoning out and slightly ignoring the rest of the song. With the UM3x I find that it comes naturally and that I'm simply presented with all the fine details of every instrument without having to concentrate on them. I don't know if that makes sense or whether that's something that could be corrected in the ER4P with equalisation, but I enjoy the experience more with the UM3x.
- Soundstage I can't really comment on, since I've never experienced a pair of phones that has made me feel like the sound is coming from anywhere but slightly behind my eyes. Maybe that's because I'm used to speakers, maybe my choice in 'phones (ER4P/ATH-M50/UM3X) all don't stage very well, but I've not noticed much difference.
Either way, I don't think you're going to be regretting the purchase at all ;)