If during your performance you start to hear distortion coming through your cabinet, and it is not because you kicked in your overdrive pedal, there are a few things you should investigate.
If you are using an active bass, check your battery. A low battery can cause strange sounds such as crackles, ear-piercing squeals or distortion. If the battery is low, replace it.
Make sure your cables are in good shape. Bad connections can be the cause of jarring cracks and pops to come through the speakers.
Make sure you aren't over powering your speaker cabinets. Even if your cabinet is within the minimal impedance range of your amplifier, sending too much power can damage the speakers. If you see connected loudspeakers moving excessively, turn your system down immediately or use the equalization and/ or compression controls to reduce the amount of subharmonic (extremely low frequency) signal.