Welcome to the forum and Sam, you'll find the members on here are a friendly and helpful bunch 🙂
my advice to your first question is to ring Audi to see if your car is one that's affected and to see if Audi did any recalls. And then see if your car has or has not had a recall if applicable.
whsts the service history like on your a4?
in terms if the amp, my advice would be to wait until you've upgraded your head unit before adding sub and amps etc. Better to do it all together in my opinion.
cheers
Steve