Hey there,
I got myself the Nav Prof. option and no Audio upgrade. IMO the sound is OKish, whe listening to pop or electronic music at normal volume. Opera's classic music and live concertes might be lacking quality. Also if you wannt really good sound hat high volumes the factory radio is not way to go
But hey it's just 4x 25W
You're assumption are right: The Nav-Systems do have the Radio Prof. Backend. This includes two Subwoofers under the front seats, two speakers in the front doors and two speakers in the rear (next to the trunk "cover").
The optional HiFi System would be equipped with two additional front-speakers in the doors, but on the heigth of the side-mirrors. You can see the casing for them. The HiFi System also has a 130W (?) analogue amp.
The H/K adds two more speakers in the rear and one center in the middle of the dashboard. Also all the speakers are of superior quality compared to the other options. The H/K is also equipped with a digital amp with EQ.
bye
Darky