"Our German-engineered and manufactured TrueResponse driver ensures exceptionally natural reproduction. A large part of its very precise transient response comes from the unique combination of materials in the diaphragm together with its high inner dampening. So when the signal stops, the sound stops too. This is how the IE 900 can reveal details in tracks you thought you knew inside out."
Ok, they say not exactly, what you've asked for; but, if you think it over, it's just evident, different sound quality needs a different quality driver as well. The drivers, and the housing of the IE900 is a further development of their former flagship model, the IE 800 S. The IE 600 plays in a different league again, it's not a high-end model.