I wouldn't consider Grados a progression of the A900 sound either. If you are strictly speaking of what people might consider "better", Grados has a few more headphones to throw into the mix. If you are considering the "characteristic of the A900 to the next level", then the Grados aren't it.
Besides a very different sound signature, Grado's soundstage is vastly different than just about any other headphones that I've ever heard. Grados has a soundstage that starts from the "inside-out". It feels like the sound is coming from the middle of your head and fading away as it gets further.
All other headphones that I've ever tried in my life has a soundstage that mimics something closer to reality... sound is emmitted from a source outside, and gets closer and closer to me.
This is a characteristic that's both interesting, intriguing, addictive, but also repulsive about the Grados. You can get really used to it and finding it addictive, or you may find it completely lacking and get annoyed with it. It's just a matter of personal preference.
Grados do not have the same type of 3d imaging that the A900/CD3000, even the Sennheiser HD580/600/650 is able to achieve either. Grado's 3d imaging is more compressed and flattened.
Anyway, not to say that Grados aren't good headphones... just that if you want a natural progression of "what's next" after the A900, the Grados aren't it.