If you are not sure if your subwoofer is blown or not you can use a multimeter to test and see if the subwoofer is really blown. Multimeters are one of the best ways to completely verify if the sub is blown or there can be any other problem that is causing distortion or another malfunctioning. Subwoofers and speakers blow when over-powered or if they continue to work for a long time above the RMS power.
How to use a Multimeter to check if your Subwoofer/Speaker is blown:
We are basically checking the Resistance of the Voice coil. The condition will decide on the functionality of the Subwoofer/Speaker.
Turn off your subwoofer completely and unplug any power source
Remove all audio inputs or any cables to make your subwoofer free
Remove the subwoofer from the enclosure and place on a flat surface
Attach the positive multimeter probe to the positive terminal of voice coil of your subwoofer
Connect the negative multimeter probe to the negative terminal of voice coil
Turn on the multimeter and you will see the resistance
If there is no resistance at all, it means that your voice coils are damaged
When the reading shifts abruptly, that means your voice coil is damaged completely. It is actually a strong indication of a blown voice coil.
If the reading is above 1.0 ohm, your coils are safe and sound and the problem may be somewhere else.