Subwoofers have no power, only amps have power. The sub is an accoustic
motor that runs on fuel called wattage. No car amp puts out it's stated
power. It has a power supply which supplies the wattage as the subwoofer
"asks" for it. Subwoofers don't, as a rule, blow from too many watts per
channel. They blow from not enough wattage per channel. If you wan't to
minimize and make things last a long time, run the woofers at four ohms or
two ohms but not at one ohm. P.

a) the impedance of the sub has nothing to do with its longevity.
b) it's impossible to blow a speaker by delivering too little power.