How much preamp will improve

I do not know about your processor, but I have used two fairly good preamps with my older Lexicon HT processor, since I am more into the two channel for music. The Adcom GFP750 (mine had black gate cap upgrades) sounded quite good for what they cost, as well as having a passive option. The best I have used is my Muse Model Three Signature, which sounds great. It has individual gain setting for 5 inputs, balanced and rca inputs and ouputs (fully balanced design), and is considered a "reference" product by Muse. I have stumbled on two of these and currently have one for sale here, if you are interested. IMO much better than the Adcom. Other mfg's also make good preamps with HT pass through, such as McCormack, BAT and others. If you don't want to spend much, start with the Adcom for about $650-$750 since you can try it and sell it for about what you pay, if you don't like it, or to upgrade.

What's wrong with the AR? What do you think it is doing to the sound?

Kr4: It's only a suspicion or a guess from my part. It usually sounds fine when I am using the Nad541. When I am using the Cary303, I feel like the 303 stresses the system a bit. And the only thing I can think of is my preamp stage is not up to the task.