fyi: if the previous owner installed the 6x9's on the rear deck and just used the stock speaker wire that hangs there, that wire ties into the rear door speakers, which will in turn draw from the rear speaker output of the radio, and ultimately sound like s***.....because there's 4 speakers on what's only supposed to be used for two. mazda used the same wiring on all of the proteges (mazdaspeed, base, mp3, p5) unless you're going for a massive sound competition car, the doors and tweeters, along with a sub of your choice is enough.
i've always been a fan of JBL, but i have all kenwood in my p5 and its crystal clear. i dont recommend mixing brands, they never sound great.