The head unit is the amp in a 6 speaker set up. It is possible to add speakers into the door blanks, which should be prewired in a 2003 and the speakers are the same as the ones behind the headrest. Upgrading to the standard 10 speaker “hi-fi” is possible but requires finding a power amp for the boot and bass speakers for behind the seats. I am not sure if the wiring loom is pre installed though, but they do come up for sale from time to time. Upgrading to the “top hi-fi” DSP system seems to be virtually impossible.
There are plenty of threads on here dealing with aftermarket upgrades if you don’t want to stick with OEM.