Been wondering this myself. I just have one cat, but I've been worried about him eventually damaging the woofers with his claws. I was going to leave the grills on all the time to avoid this, even though I think they looks better with the grills off.

One thing I thought of was taking the grills off and spraying them with Lemon Pledge. Cats hate citrus smells. May work to keep them away from the grills.