I have both a 900 and 900A rack, and any differences seem very slight. The 900A is the newer one, and the circuit board traces are a bit different than the old. I seem to remember reading somewhere that the old 900 rack didn't work with one of the cards in the last slot (or something like that). Maybe the two slot card for the flange, or a stereo card like the 929. I'm really not entirely sure, but that is the only major difference between them that I've ever heard about. Maybe an email or call to DBX would clear it up.