Went to a few car audio places over the weekend. There seem to be a couple of options. First option is to just buy an amp which could make a reasonable difference - amp would fit under the seat and costs about $500.
For an even better result, the next option is to then upgrade all the the speakers in the car for about another $500. The factory speakers are quite small, 5.5", despite the speaker covers on the door looking much bigger. So for about a thou, you can achieve quite a significant audio upgrade. I'm still in contemplation mode on this....