I'm a novice at this HT stuff so I'm a little confused at how everything works if I added the 200's to drive my mains for listening to two channel music. So, based on my limited knowledge, here is how I think it works.

I would connect RCA jacks from the 200's into the left and right main speaker preouts on the Denon. Then I would unwire the main speakers from the Denon and rewire them to the 200's. This looks like that's all there is to it. All of the output controls and processing would still be done by the Denon.
Does this sound right?

I could then listen to two channel music with the mains powered by the 200's. However, it seems that the 200's would drive the mains in all modes including HT in this arrangement. Is there any way to just use the 200's for two channel music and still use the Denon for HT?

Finally, does it even make sense to add the 200's. My mains are B & W 604S3's. I would think that that going from 110W to 200W would give them a fuller sound.