Burned or open coils are not covered under the manufacturer defect warranty. It takes power well beyond the subwoofer's power rating to create enough heat to burn voice coils. It may be an honest mistake, but since the subwoofer wasn't used correctly we can't replace it under warranty.
Any time you buy subwoofers, match the RMS power rating of an amp to the RMS power rating of a speaker at your wiring impedance load.