Polk subwoofer making crackling noises

I have a polk subwoofer that will distort if the bass is too bassy. It used to run fine with nice tight bass then after a couple of years started to distort. It still sounds good for soundtracks but when it comes to certain very bassy parts of the soundtrack it will cut ut making crackling noises which irritates me. So what do I do? Can this be fixed ? I paid almost 400 4 years ago. Does this mean it's time for a new one or can this be fixed?