Puh, define 'balanced sound signature'.I have just bought the IE600 and have the Neon Pro, overall I am entirely happy with the Neon Pro bass with the switch off, with it on the bass quantity is more than I want and more than the IE600 I think, I haven't done a back and forth comparison.
I haven't heard the IE900 to compare.
Overall the Neon Pro is simply better balanced than the IE600. The IE600 has elevated highs that give a sense of sparkle, cleanliness and spaciousness. That is all well and good but the trade off is that they become sibilant on certain problematic vocals. The Neon Pro is detailed in the highs but not as elevated so manages to stay just south of sibilant on the same problematic vocals.
I know the 600 isn't a 900 but from what I understand the 900 is tuned to sound technically impressive rather than truly being a balanced sound signature.
I hear the IE900 as less fun than the NP but to me it is the more balanced one in the sense of less coloured and monitor like...