Joe
Unless the Mitsu has volume control built in..Do Not Try hooking it to your amp. The older Sunfire Theatergrand II is a Bob Carver design and brings with it some of the Carver sound and features which you may like. (I own this older model myself).
You can find these used for around $1,200. This unit does not have all the latest bells and whistles, but does have all you may want or need. I mention this unit only in the context of you wanting to stay with what you seem to like and my guess is that the Sunfire would give you what you want and even better sound quality. Carver likes to voice his SS gear towards what he considers (tubed like) in sound. While I've never listened to your pre-amp nor do I know it's features..my guess is that the Sunfire may share some of them.
On the other hand, you could pick up any number of used pre-pro's with DD/Dts decoding for around $400-$500 here on Audiogon and this would also allow you to keep your amp. As a matter of fact, you could even keep your Carver pre-amp in the system as a two-channel pre and simply let your Pre-pro's signal pass through it when watching movies.
As far as the units you asked me to comment on, I don't have any experence with them so hate to comment. You could post a thread on them, others may have used them in their HT systems. Hope this helps a little.
P.S., Check the back of your pre-amp and see if it has a DB-25 computer type input on it. A very few products have had these as 5.1 inputs. Onkyo had this on some of their older stuff and Sunfire (Carver) have also used these at times. My Theatergrand II uses one also.
Dave