Yeah, it would be nice to upgrade the sound, but it's not in mine at all (9-speaker Sony) - I mostly find the plastic door panels vibrating at certain bass frequencies spoil the system - the doors just aren't braced well enough.
In my previous Kuga, I tried wedging some stiff foam blocks into each door pocket, which works really well for a few weeks, until the foam relaxes to its new shape, and the resonances came back. I gave up redoing them every few weeks.
I think mounting bass drivers in plastic doors will always cause problems, until they are really stiff and well braced.
Several years ago, I had a BMW with a premium Harmon Kardon system in it, and it was amazing, as it had two 6" subs on the little parcel shelf and thus used the entire saloon style boot as the bass cabinet, so would go really low. The bass response actually changed depending how much stuff I had in the boot! The emptier the better.
A friend of mine has a BMW 1 Series, and it has a sub mounted under the driver's seat, but actually in the metal floor panel, not just a plastic sub fitted and he gets zero resonances and solid bass.