Theta standalone DAC vs. Casablanca DAC

I have a Theta Carmen connected to a Pro Basic IIIa DAC and an Adcom GFP750 preamp. I thought that sounded better than the DAC and preamp in the Cassanova. I'm using a DreadnaughtII and Snell speakers with a Rel sub. Some questions:
1) How far up the line in the Casablanca DACs do I have to go (standard vs. superior vs. extreme) to improve over the IIIa?
2) How is the preamp section of the various Casablancas for 2-channel (I might go with an older CB-2)? Better than the Adcom I have now? I'm probably going to connect all video sources directly to the TV, so I only care about the CB's sound processing.
3) If I wanted to upgrade my 2-channel preamp in the future, is there a way to use the DAC in the Casablanca and then route the signal directly from that to an external preamp, bypassing the preamp section of the Casablanca? Or would I have to get a standalone DAC again?
Thanks everyone. -Dave