Ummm ya and how do you think this happens? Without sufficient cone movement for a given power the coil(s) will fry. This also can be effected by ambient temperature because a motor that has been sitting in a black car on a 95 F day will die a lot quicker than the same on a 0 F day. So a sub in a proper enclosure doing a 40hz tone will last longer due to increased cone movement/cooling than the same on a 60hz tone. Just a thought...
Polo.