I have the HK upgraded sound system, which has subs under the seats. I do like my music loud, I think the bass was responsible for loosening the panels on my car...
However could be wrong and may just be the design on earlier cars that has now been rectified
Current: F10 530d M Sport, W176 A200 CDI AMG Evolution. Previous: E70 X5 3.0 SD M Sport, F20 120d M Sport, E81 120d M Sport, E46 325i M Sport, E46 318ci