I think for the c-4's you should definitely get the Ref-150. I am currently running the C-1's and I sometimes run out of juice for those with 100 watts per channel. I wish I had 150 to 200 watts per channel with my C-1's.
ARC Ref 75 vs REF 150
Why in the world did AR not put any meters on the front panel of the 150's? Do they hinder the sound quality in the 150's? I'm torn between the 2 but it makes more sense to spring for the 150's so that i have the extra power. However from reviews online, it sounds like 75 watts is plenty. My room is 20x24x 8ft tall.