Where to start:
9 speakers.. 2 on front left, 2 on front right, 2 rear left 2 rear right and a sub?
what exactly sounds bad? What don't you like? try to quantify those issues first.
Then, look at the JBL MS8 or any of the other system integration processors. You keep the head unit, bypass the factory amp and add a subwoofer amplifier. Only issue is and they may have it worked out is the bluetooth may echo. Has to do with the latency of the processing in the MS8 if you use that.
The MS8 has an auto tune capability (Logic 7) that does a very nice job making a system sound better.
Anyway, that's where I would start.