Like probably many in this thread, I have tried many IEMs over the last 10 years, including custom (hard shell) 3-way IEMs, Triple 10's, Klipsch 10 and 11's, and more recently the lower-cost Kz ZS6 and AS10. I thought some of these prior IEM's sounded good/OK, but when I finally decided to try the ER4XR, I have to admit that at first I was disappointed - they sounded so different.
I then started going through my favorite songs, and listened carefully, and I realized that I could hear things better, and wind and string instruments sounded "true", like I remember growing up listening to guitars, etc. during a live show. That is then I finally realized how much coloring I was used to hearing with the other IEMs. So I started back to back sessions with specific songs where I had a pretty good idea of how a particular instrument sounded "live", and I was also trying to hear how much of the singer's voice was coming through. Man, what a revelation. None of the other IEM's that I have come close to that "even" and true sound I get from the ER4's. In fact, for some songs that are not recorded "well", it shows how crappy the song is!. And for some other good recordings, they sound sublime. If anything, that is the only negative I have about the ER4's - they "will" let you hear bad music more clearly
I pretty much don't listen to any other IEM anymore. I use my FiiO BTR3 with all IEM's, since my S10 supports the good codecs, and the built-in amp in the FiiO seems a little more "punchy" than listening to the ER4's directly from the S10. Maybe it is part of the whole DAC + Amp on the FiiO that makes it better. I don't know.
I recently signed up for the Massdrop on the ER2 so that I have a second IEM to use, but I went with the ER2SE this time, since the freq response is already a little bit "stronger" on the bottom end compared to the ER4XR that I have today. Looking forward to reviewing those soon