Comparing the three (lucky for you - I've owned them all and like them all) - the HFI-2400 are darker and much bassier, the DJ100 are very mid-centric and n-shaped, the PRO2900 are more forward, spacious, bassy, bright, and extended. The 2900 imho are better than the DJ100, but if you don't like the more forward/aggressive sound or the coolness then you may not feel the same way. If you want to "grow up" from the DJ100 though, the 2900 are not the choice (they're the "grow up" from the 2400). The "grow up" from the DJ100 imho will come from Sony or Grado. Very much out of the scope of this thread. And remember I haven't heard the Signature Pro - for all I know they would also be that headphone.
that sums up the 2900s well (at least the way I heard it).. that's why it wasn't for me personally.