The problem with multi-driver IEMs is that there's usually a high price tag for the custom fit options or there's not enough tonal balance because more drivers shifts the focus on a particular frequency band. However, multi-driver IEMs tend to sound more energetic and they sound more intensely involving. They also tend to seem to sound more powerful by immersing you into the music more deeply. They're more vivid. It's nice for a while, but it get's tiring after long listening sessions. The Ue-18 pro is excellent, but the bass and mid-bass hump makes the music sound slightly unnatural. It gives the sound too much bass power that I don't find in live rehearsals or live music.
I find the ER-4PT/S combination to have the most accurate and correct tonal balance and the bass is well textured without sounding overpowering. They don't impress most people that have heard mine because there's no "wow" factor with them. For me, I am continually impressed by them because they get the music and sound just right because they are so accurate. I want the music to impress me not some piece of audio component. I want as much clarity and transparency to let the music flow through.
I'm thinking that the Ultimate Ears Ue-900 and Westone 4R don't do this, but I never heard them so I wouldn't know. And, I'm not about to find out only to be let down by their sound compared to my ER-4PT/S.
Trying to find audio components that achieve this level of sound performance gets to be very difficult and quite expensive. I'm trying to find the most direct and least colored sound possible and I find that reference grade components are the only way to go for me. My idea of reference grade is very different from what is available on the market now. I define it in terms of accuracy and transparency. In other words, I'm searching for the least colored and least impressive "wow" factor possible. Most companies can't make a profit by selling products that cater to me because audiophiles are looking for a specific house sound or model that delivers a unique sonic picture.
It's nice to have a fun sounding consumer grade pair of headphones or IEMs, but those get sold to someone else quickly for me in the past.