I am relatively new to IEMs but I have completely fallen in love with them, and so I'm looking to improve on the ones I have.
I currently own Sony XBA-1s, and while it took some getting used to, I have fallen in love with the balanced armature sound. I particularly like the slightly forward vocals, and fast, articulate bass. However, they lack treble extension, and certain female vocals sound muffled, suddenly dropping in the mix when they hit certain notes (usually higher ones).
1. Great female vocals. This is the most important. Any lack of detail would not be acceptable here.
2. Orchestral and acoustic instruments. Accuracy here is also a must.
3. Instrument separation and imaging. I consider myself an analytical listener in that I want to hear every instrument, and pick out the details if I so choose.
I listen to orchestral music, classical guitar, metal, and trip-hop, all dominated by female vocals.
For a sound signature, I like fast, articulate bass that is neutral in quantity, smooth and neutral-ish mids (vocals slightly forward), and extended and detailed treble. Not necessarily bright or forward, but anything that can be considered to have rolled off treble probably doesn't cut it.
Other than that, I prefer balanced armature to dynamic drivers. I tried the Hifiman RE400 and found the low end too slow and lacking articulation for my liking. Also couldn't get a good fit with the standard bi-flange tips, but thats another story. I've also tried the TDK IE800, and found it too V shaped, with recessed and dry vocals.
So go ahead, give me a suggestion.