When I had my mustang with a mach 460 system in it, I switched the whole thing out to run a pioneer 6X8 setup. I was extremely disappointed in the high frequency sound coming from the center speaker tweeters. They weren't the top of the line pioneer door speakers, but they should have done better than they did. After installing bass stoppers in the doors and re-attaching the tweeters off the mach 460 to the new aftermarket setup and a sub in the trunk, I had one of the best quality car stereo systems I had ever heard. The pioneers were monsters in the high mid, mid, and mid low sections, the tweeters acted as horn drivers would on a PA speaker, and the sub filling out the lows.
Point of that story is that if you can find some clean fitting tweeters to put on your door corners by the mirrors, they are well worth it to add to the quality of sound in the car. Sub or not, they make a big difference IMO. find some quality door speakers and some tweeters you can run with them. Can do that all off a head unit as well, no external amp required if you aren't running a sub. Only place in the truck it won't be loud enough is the third row.