any speakers with a higher RMS rating than the source unit you are using can cause the speaker to be under powere. you can actualy blow more speakers "under" powering speakers than over powering them,
so running the 6x9's from the head unit isnt the best idea....
if you amplify the speakers you also gain another benefit...
the head unit can only proccess the data given to it at a set volume, any higher up that volume and it starts to distort the amplification, if you use the "pre-amplified" outputs on the rear of the head unit you can use a stand alone amplifier to clean up and then amplify the signal.... so you can not only play the sound louder, but clearer..... as demonstrated in my first car, i had 16 year old paper cone speakers amped up and ran them for ages sounding brilliant,.....plugged them into the standard head unit and they sounded pap!!!
lol
but whatever you do, dont throw away our advise cos its not what you may have "seen before"