How would I go about upgrading the audio in my 2005 Audi S4 convertible that DOESNT have Bose sound system?

I am a complete noob to car audio (aside from installing a new radio)

Basically I want to upgrade the sound quality so it isnt always having to compete with the road noise because its a soft top. Also maybe a subwoofer if those will even work in a convertible.

Price isnt an issue because im just looking for knowledge at this point.

Thanks for the help!