From my experience, our stock 6x9 is good enough. I would highly recommend you keep them.
I upgraded mine to Polk Audio and they hit so hard, the rear deck permanently developed rattles. I was about to return them if it were not for the fact that my 6x9 came with tweeters. The tweeters really brings out the extra sound from the rear so I kept it. I also had to dynamat the whole rear deck just to reduce, not eliminate, the rattle.