In my opinion, German engineering and quality is second to none in most things, however in my experience, car audio is not one of them
. If you have the factory Blaupunkt or Becker, that could be some of your trouble. It won't have much in the way of power. I don't know what kind of speakers you installed, but there's a wide range of quality available there too. A visit to your local car audio shop or Circuit City, etc. should be enlightening. Don't be afraid to ask lots of questions and do lots of listening. Crank it up if you want to! That's what they're there for! Hopefully, Bell will chime in (pun intended!) with some suggestions.