My re-400 haven't arrived yet so i had some time fooling around with the UE900.
If you take a look at rin chois blog http://rinchoi.blogspot.de/2012/10/ultimate-ears-part3-in-depth-analysis.html
The UE900 have emphasised upper bass and veiled lower mids, which for example if you listen to Metal makes the Bass Drum and Bass Guitar a little bit bloated and decreases detail in the mids (e.g. there is a guitar solo but the low frequencies bleed over it). By blocking the Pinhole and adding resistance to the signal path, the Bass decreases and the mids increase and you end up with a nearly perfectly flat IEM.
after modification it looks like this:
I can confirm the effect, although it is not as much of a difference as it looks on the graphs, and i look forward to comparing the modded UE900 to the re-400!
I took the graphs from rin chois blog which is dead but if i should remove them, please tell me.