Ok thanks for the informations.
Harman/Kardon give me a very good sound in the mid/high frequency, but when I push the sound at a high level, the bass "crack", and the plastic of the car vibrate, I don't like that.
So I just want to add some more bass with a subwoofer (phoenix gold X-max 15") and filter the Harman/Kardon system.
Thank for you infomration, now I am able to do waht I want ;-)