Thanks for all the advice, particularly Mooncricket! Lots to read!
Carey, I am looking into ordering a newer trim piece inside that would fit maybe up to 8 speakers instead of 4. I think those are then 6" speakers, but I'm not sure. It wouldn't be terribly difficult to fit 6" speakers into this surround either.
The current speakers are JVC, but are paper cones, and sound like crap.