well it depends how loud your cranking it, if it's clear and it's sort of loud then your just pushing the speakers too much, but if it distorts at a fairly low volume then your speakers are blown, caused by cranking them past there limits.
If you get new speakers turn on the radio, or cd and increase the volume, until the music stops getting louder and just more distorted, now bring the volume back down to the notch where the speakers are at there loudest and still remains clear, and do not exceed that volume, ever ...
the radio can usually be turned up further then a cd because of signal strength.. for example... with my stereo 26 for CD's and about 35 for radio you just have to listen to the speakers.
hope that helps
steve