I do want to help a bit with choice between the UE 900 and 4.Ai, as I feel they are just about evenly matched (although all won't agree with me) with different genres.
- 4.Ai: Rap, Electronic, Rock, Country (male singer)
- UE 900: Country (female singer), Pop, Alternative, Classical/Instrumental/Acoustic
Now, does this mean I recommend the 4.Ai for rap and electronic? No, it just fares better over the UE 900 :p I would recommend you get the 3.Ai for rap and electronic :p
There are obviously some exceptions to the general idea though. I know I've stated signature differences enough, but I do want to reiterate them quickly if anyone knows what they already want an an IEM. Neither are neutral, and I still have no clue what natural means :p:
- 4.Ai: Mid-bass focused bass with a strong impact and decently strong texturing. Midrange that focuses on the lower mids while creating a relaxing, lush sound with vocals and instrumentals alike giving it a warm touch overall. Highs are linear with a high-treble focus with a dip in the lower highs.
- UE 900: Linear bass focusing all around with a tight punch and ample impact with good depth for fluid sub-bass reproduction. Midrange that focuses mainly on the upper-hand without loss of detailing that has sweeter vocals and clarity. Highs are well-extended with a strong mid-to-high treble focus with a dip in the lower highs.