Unfortunately, a better driver will yeild unpredictable results, because the cabinet is likely tuned for that driver - you can pull it out and see if you can get any specs or a model number off it to find specs, then at least you know what to look for in a replacement driver. Replacing it with a really nice driver usually doesn't yeild the best results, and by the time you are done with it, you could have just built a better sub yourself from scratch.
Upgrading the amp might help, but finding another one that fits without modification will probably be impossible and also may not yeild the best results either if there is some kind of boost built into that one, tailored to that specific setup.
