Personally, I'd leave the 975 on Native then you know it's not changing the video colors. So that leaves the Oppo and the TV as sources of possible color changes.

It's possible your old processor was treating the reds different then the neutral 975 and now that you switched you're just seeing the effects of your old processor. You'll probably just need to re-tune the color setup on the TV and possibly the Oppo to get the colors right. What reference disk are you using?

Aside: I bet the 975/5000 combo makes those Paradigms sing...