Reading thru the thread, I just want to give you a few thoughts and my opinion.
Starting with the TV, You are sitting back to far to get you into a movie. I have a 65in in my main, and I sit 9ft from that, and I feel it is perfect for bringing a person into the movie.
In my bedroom, I have a 40in, and I am about 9ft from that TV, and I understand how you feel about the size. I want to upgrade my bedroom TV, although I’m not sure why, because all I do is fall asleep to the TV/audio
Also, in my bedroom, I have the Klipsch f-2’s with the C-2 center channel, which are the older model of the f-20’s. Years ago, I was using a f2’s and a C-3 in an apt, and that set was great for movies.
I am also using an Onkyo with the Klipsch, and I feel they make a good combo.
So, if you can, sit closer to the TV until you get a larger one. Get rid of that Polk sub, if you can afford to spend $500 for a sub, look at the SVS PB-1000. The sub can go as low as 19hz, that is around the range you would want for movies, of course lower is better, but for $500 you are not going to beat that price on a sub hitting that low.
To answer the original question, get the C-20, because you always want to timbre match.