Good luck finding speakers that matches BMW's amplifier outputs. Most home/car speakers are 4 or 8 ohms. BMW likes to use odd-ball impedance speakers like 2-ohm, 6-ohm or 10-ohm
IMHO, the speakers in our cars are pretty good, it's just that stupid amp programming to under drive the subs in the coupe that is just plain awful - don't know what Carver was thinking!