if your gauging efficiency by creation, and subsequent loss of, heat,.. I can see how you can twist that around... efficiency, if i recall correctly, is the output of power from the amp to sub vs. the amount of heat it produces while creating said power with a specific rated input... say... with 14 volts you are amping it up to 100 W. (not in the real world of course) if you can output 80% of that power to the subwoofer then 20% is lost to heat hence it is 80% efficient.
All the winter is doing is keeping the lil guys in their cooler than in the summer time. I know someone out there can explain this better than me... please go ahead.