I find it funny reading reviews.
UE900: Improves and perfects everything TF10 was lacking.
W4: Improves and perfects everything W3 was lacking. And we almost end up with the same sounding type IEM. Having said that I have to think there will still be their share of listeners that will outright prefer W3 or TF10 over the new ones. Bass is big in peoples minds and bass weight is LESSER on both IEM's.