Not sure the basic system is any more simple other than it has no amp in the boot. Have a look here:
https://www.volvoclub.org.uk/tech/S4...Electrical.pdf
Pages 10 and 27 detail the components all linked by fibre optic MOST.
Basic (Performance) audio will have at least and ICM, MPM & AFM. The main difference between Performance and High Performance is the addition of an external amplfier (AUD), the MPM is normally 6 disc instead of 1 and the rear doors have tweeters in them.
The best upgrade (in my opinion) ou can make to the performance system is to fit OEM tweeters to the rear doors to make the audio system 8 speaker. We did that on our V50.