I haven't heard the PRO 900 or the SR225i, so I'm afraid I wont be of much help there, but going by what I know from other PRO and Grado headphones, wind016's suggestion of the W1000x seems pretty solid. Between the Edition 8 and 9, the highs and lows of the PRO 2900 seem closer to the 9 to me. It'll be interesting to hear if these Signature Pros are close to the 9s in their sound.
The PRO900 compares almost directly to the 2900. Imagine the 2900, but closed and all that it implies--not quite as airy, probably a 20% (qualitative estimate) increase in bass quantity, and slightly more sparkly highs. The mids are a little more recessed on the 900 due to the highs and lows being more pronounced--they're the same driver, so their FR are actually pretty similar. Of these, I've actually heard both and probably would have bought the 2900 if I could use open cans.
Reading reviews on the wood AT's sounds like I would have the same problem I have with my 225i--lack of the bass extension I'm looking for. I considered the D7000, but I'm not sure it has the top-end excitement that I want.
Sadly, I don't have anywhere I can demo any of this, as I'm currently deployed to Afghanistan. I've been basing my purchases thus far entirely on reading hundreds of reviews...which has worked out "ok".
Nope. See above. Have you heard the PRO900? I'm not being facetious; the opinion in the PRO900 thread is very polarized, but the consensus seems to be that while the Pants of Thunder deliver some pretty decent bass, it's comparatively muddy and it's more of a midcentric can, being based on the TR50P.
Back on topic, though, it seems like no one in here has heard both the PRO900 and the Editions. I may have to go looking elsewhere for opinions. I'm gonna stick around this thread, though...i'm looking forward to opinions of the ProSig.