Changing to a better head unit would be a good start, or using an external amplifier that doesnt distort the signal like the cabasse unit does.
Then maybe upgrade the front speakers to focal or rainbow (or something high end) size is not that important if you go for a good quality speaker. I have 13cm rainbow components in my 182 and they produce a really good sound. Try and ignore most of the rubbish that your local halfrauds or roadradio stores sell, its not about "quality" at all. The better stuff you probably wont have heard of before, get yourself on talkaudio.co.uk, there's lots of info and reviews on there and plenty of helpful members.