Thanks for the answers. I found them. Put on some music with max bass but like BenN said: Can't say I've noticed anything coming from them, we'll not noticeable anyway! Upgrade frome standard was about €400... Standard must be very poor.
Alpine retrofit won't work with code 676. I found these (dutch websites)
http://www.mbcaraudio.nl/compleet-so...0-9-15200.html
http://www.hifimotive.nl/product/384...tuigspecifiek/
Like to hear if someone has found a custom fit.