i would check the install, tho
it's possibly mechanical noise due to vibration, poor damping, etc.
if it's distortion from the tweeter, it's most likely overpowered already
it's hard to say unless you're positive it's actual signal distortion, and not some sort of 'noise' or added artifact
the only reason your head unit would distort is if it were having voltage problems... maybe check the wiring of your head unit while you're at it, it's a cheaper temporary solution than buying an amplifier that still may not correct the problem
i'm not ripping on the speakers, i have no experience with them, but "distortion" is pretty hard to narrow down without actually hearing them, and i can't imagine it's being caused by the head unit, since the preamps and amps are specifically made for each other, and clipping should be a non-issue