Question on speaker quality??

I have 2 sets of pioneer 6 1/2 3way speakers in my civic. One pair is about a year old but in perfect condition, max watts like 160, and the other pair is new, max was like 200 watt. I think it was the same model, just different year. Anyway i have a 8400 pioneer head unti, 50*4 right? So how come if i have the bass up on the speakers, and turn off the sub, the speakers will start to sound distorted at about half the peak volume of the head unit??? It doesn't make any sense to me, since they'd be getting nowhere near what they could handle.