That head unit won't push anymore than 18WRMS per channel so I'd say amp your current speakers. Most speakers have tweeters mounted on coaxial anyway so more tweeter won't sound as good as a balanced, amped set of coaxials, which you already have.
Besides, tweeters are the least appreciated type of car audio speaker in my experience.
2007 Cobalt Ls Manual Coupe
Kenwood Excelon X996, RF T3 Components, RF Power 6x9s, Sundown Audio SA10, RF Power T600.4, Alpine MRP-1000, Kinetic HC-1800, Black headlights, FX-R HID 55W, Custom led tail lights, Led reverse lights, Airbox mod.