Upgrading Fosgate Sub / Low Profile

I bought an 03 XTerra SE with the Fosgate system, and plan on doing some upgrades, but can't find any really good info on the systems setup. So two things:

1. I plan on adding an RF modulator to get an input (for rear seat DVD) to the Fosgate system. Is there a better / higher quality way (without putting in a new headunit)?
2. Not a big fan of the Fosgate Sub in the rear but I like the low profile install. What would be the best route of upgrading? Swapping the sub with a better one? How many watts is the sub? Should I run a line level converter on the input, and put in my own amp?

Thanks in advance!