Can i keep the factory deck and upgrade everything

I own an '03 Audi A4 with the factory Audi Symphony radio (not BOSE upgrade). The deck is an AM/FM 5 disc single load CD player which looks really nice with the rest of the controls (especially lit up at night) which is why I am reluctant to replace it. The problem is the power on FM is barely acceptable and on CD is horrible. Can you suggest a setup that would allow me to upgrade the power yet keep the factory deck? Maybe an amp and a 12" sub to give the system a kick? My goal would be loud, high sound quality for all types of music (Alternative, Metal, Rap, Classical). Thanks for any advice you can offer.