Nevermind, I'm a humongous idiot. I only got 3 hours sleep last night.

On my TV, I had set the other three video modes to "Skip." I had to turn the comp video input on.
I had the same problem with my cousin's setup though (RX-V730) and we still can't get it to work. All of his video modes were turned on. Connections were done just the same, inputs selected properly in the menu, but we got sound, but no picture. Strange. Cable box and DVD both wouldn't show a picture. He's just got it going straight through his TV now though.