I have heard the CA900 and CA 906 and I liked them both. The CA 900 had no remote and the CA906 volume is painfully slow. Sound wise the CA906 is awesome, think Audio Research territory and it is capable of that goose bump feeling of reality too. The CA900 is a bargain for the money, it is very very good and at that price I don't know of anything that can touch it but with the CA906 you get more of everything. With the CA906 you are comparing it with some serious top of the line preamps now. No idea on the C900?
Rob