[Neil Green[_2_]]

Hi all
My son bought some 6" Kenwood speakers from a local
shop, not cheapies, and we installed them on the rear
parcel shelf of his car on Sunday.
If we crank the volume (to around 75%) and bass up the
distortion is really severe.
You can see the cones bouncing up and down and they
make a vibrating sound which is very unpleasant so he
has to either lower the volume of reduce the bass to
eliminate this noise.
What I'd like to know is if this effect is just a
limitation of 6" speakers or whether the speakers
themselves aren't up to par or if there's any way of
reducing this distortion to acceptable levels.
Would housing the speakers in boxes placed above the
shelf help?
Thanks
Neil.