stock stereo pitch problems?

[yay for cars]

i have just the stock stereo in my P5 right now and i noticed that when i turn it up, the pitch of the music seems to get a little higher. i wasnt sure if this was a form of distortion or what, but my question was does it have to do with the small amp or is it the actual head unit? i was planning on keeping the stock head and putting it to a 5 channel amp, but not if this pitch problem is beacuse of the head itself.
thank you

 

[1sty]

The speakers become unable to increase the lower frequencies as easily as the highs as you raise the volume so the low end naturaly begins to break down. I'v seen it just about all the speakers I ever owned.