Replace or Upgrade Boze speakers/system

[ajdo]

Hi All... Do not know much about car stereo/speakers but can someone explain how I can upgrade the boze system from my CX7. Really not happy with the sound. Now that I added XM radio, I would like something that sound better!
Reading the threads I undestand Boze splits the different High/Low/Base making it impossible to just change the speakers or is it?

Thank You for your help.
ajdo