I got my UE 18 pro when it first came out and was totally disappointed, everything sounded good appart from the treble or the lack of it. As you said there hasn't been many reviews on the 18 pro so I've often wondered if there was something wrong with mine, I hardly use them anyway.
Just so happened I found a shop that had the UE custom series on demo and I thought I'd try out the 18 Pro and the demo version sounded very similar to mine, kind of treble light, not unlike the IE8. I tried out the UE RM too and they sounded very good and very balanced to me, just what I expected the 18 pro to have been
A few hours later, I found another shop that has the UE, Westone and JH audio customs on demo and I tried them out. The 18 Pro in the shop sounded more balanced with more treble but sounded very harsh and unrefined, and the UE RM sounded a bit thin, I think it was because I was not able a get a very good seal with the demoed ear tips. The demoed JH 16 sounded very powerful, very full, deep and powerful bass, even more so than my 18 pro, and have very clear and crisp treble, not unlike my HD 800, it sounded quite impressive but not with the balanced ssound ignature that I'm after. Lastly I tried the ES5, they sounded thin and horrible as if there were no bass drivers at all, I do hope it was because there was something wrong with the demoed pair as I wouldn't want to use them even if I were paid to.
Don't know what you've decided to get but for me, I'm very tempted to get the UE RM, just that in the back of my mind, I still haven't gotten over the disappointment of custom IEM with my UE 18 Pro experience.
