Why is my subwoofer distortion?
Beside this, what causes subwoofer distortion?
Over-Powering Low Frequency Speakers (subwoofers) Failures in subwoofers are caused either by ELECTRICAL and/or MECHANICAL power handling issues. Electrical failure occurs when applying to much power to the subwoofer(s) and causes the voice coil(s) to "burn".
In this manner, how do I stop my subwoofer from distorting?
For the subwoofer, you need to use High Pass Filters which removes the distortion and low bass. This enables your system to gain a clear and clean sound where you can also try lowering the crossover point for the low pass filters that bring the bass up and forward thus achieving the maximum pure sound.
If the Hz and the power being supplied to the coil in the speaker is good, the signal should not cause distortion which will in turn cause the speaker to "Crackle" or become very "Fuzzy" or "Hazy". You should be safe.