Kind of figured it out...
I started messing around with my signals and figured this out:
If I play a movie, there's no hiss. My movies are all dolby digital
When I'd stop the movie on my HTPC the his would start up again.
If I play a music video I get hiss. Figured out my music vids are PCM
Switched to directv and got hiss. It was in PCM mode so I switched to dolby digital and the hiss went away.
So I guess the real problem is that I'm getting hiss only on PCM and only in my subwoofer. Does anyone know why I'd get hiss in my sub with PCM?