I used to do this for a living, granted it was a decade ago. I would go with a 5.25 and tweet in the door in place of the 6x9. The sound quality is determined by the speaker, amplifier and head unit. If you are looking for true sound quality then I would run a head unit with burr brown dac's. If you are just wanting better than you have right now, then run your stock head unit with a converter to allow for a good amplifier to be installed. Separates upfront will allow you to angle to tweeter and mid bass to put the staging in the center of the windscreen or at your seat depending on if you just care about the quality being for you or everyone. I personally have Zapco amplifiers and they do a very good job. My separates are hybrid audio and they have been amazing since day one. If you can find some older PPI amplifiers such as the PC450 in chrome, gray or dark gray they are bomb proof! Pm me if you have any additional questions.