With that large of a room, I suppose the 6C would be a good fit, although I can't help but think you may need a more powerful receiver... I have the same receiver as you, but a 4T Hybrid SD hooked up in a traditional 5.1 setup in a 13.5 by 16 foot room instead.  How do you like the height channels?  My room is too shallow for a traditional 7.1 setup, and I've floated the idea of using the Dolby Pro Logic IIZ until my living room magically supersizes itself...

 I would write or call Aperion letting them know you want to utilize their Trade Up program, and find out the shipping costs involved.  I am about a stones throw away from upgrading my center to a 5C, and I contacted Aperion first...