Thanks jllphan for you piece of advice. It will be helpful for sure.
I do have another question.
My brother owns a 2001 330d (the ultimate Diesel rocket) that comes with the Harman Kardon system.
Is there something special is this subwoofer kit (air conducts, etc...) that helps bass to get inside the car?
I do not know the size of the subwoofers (an would like to know it).
They seem very small but have a correct kick. Way better that my CLARION active subwoofer
If there was some air conducts to conduct the bass, is it possible to buy a second hand H-K kit (to use air conducts, if they do exist) and change the speakers for better quality ones?
Kind regards from Madrid, Spain..........