To reiterate, you are looking for a bookshelf sized speaker to fit into your entertainment console, and the size of the studio 20's or S2's will fit that location best for you... Also you were looking to get the CC-690 as upon hearing it at your friends house, you Cr@pped your pants... Am I correct here...?
Seems you like the Studio's here... as do I.
Now as I have the 100's @ 690, I couldn't really answer your question you pm'd me whether or not the 20's would be enough for your Mains...
Anyone here that can add something for Bisonman.
I had suggested that he think towards the 40's, but Im guessing they wouldn't fit the space...
upon which I suggested maybe get a used set of Sig's for a bit more punch.
The more info you give us, the more everyone can help you out...
Good Luck Bisonman