I have a 2000 Celica which has a similar setup to the matrix. Start with a head unit that puts out more power than stock. Dont be fooled by companies listing the MAX wattage, always look at RMS. Putting more power to stock speakers will help with volume but not "oomph" as you stated. It will however provide clearer sound and some RCA outputs desperatly needed for an amp. Then I would suggest an Amp and maybe one sub. I'm runnging a single 12" sub in the trunk and it sounds great. built my own custom box to save some room as well.
