6x9s used to be incredibly popular. You could buy a pair of little sealed boxes and put an extra pair of 6x9s almost anywhere. Somehow along the way the market started seeing "supertweeters", which I believe might have been a Pioneer innovation. We ended up with monstrosities like 5-way 6x9s which have pods with a ridiculous total of 4 tweeters, each one smaller than the next. Manufacturers would put incredible power handling figures of 500+ Watts on these things.
Our hearing is most sensitive to treble, and there's no need to have more than one capable tweeter on a coaxial speaker. I used to run a 3-way or maybe 4-way 6x9 custom mounted in the rear of my first car, with 100W RMS on each one. That was a real ear-bleeding mistake.
Save your money and get one good pair of 2-way 6x9s. Stay away from WalMart's car audio department.