Can you describe what you aren't happy about with the factory speakers? If it's a factory defect, you may want to explore that option first because I believe you are the first to complain about the audio quality. (Usually I'm the first to complain about audio quality in a $50K plus vehicle).
That said, before replacing a bunch of speakers and potentially ending up with undesirable results, you may want to consider a more metered approach. First, if you haven't already, adjust the EQ to drive more clarity from the speakers. I find that 500-1K Hz are almost always overdriven in cars whereas the 5K+ are underdriven. Bass at < 100Hz, depending on your music choice could be underdriven as well. With the Carnival I was able to tweak the EQ enough that it just didn't warrant replacing any speakers. That and I like the centerpointe sound setting for Sirius XM. Second, if that isn't enough try adding sound baffling material to enhance the current speakers. The amplifier does seem to not have a great amount of RMS wattage, so if you do end up replacing speakers, keep that in mind. Good luck.