Sogood51, transient power peaks is also a very important parameter.
Yes, most of the time amplifiers just coast along around 5-10 watts. But even within that period, there are transient signals (in milliseconds, too fast to register on power meters) which can consume huge amount of power in short bursts. That's what the extra dBs of headroom is for.
Typically, most good power subwoofers use amplifiers in the 250-500 watts range. Some, like the new Velodynes DD use around 1,300 watts (Class D amps). If space, weight, heat dissipation (for class A/AB amps) and cost weren't a problem, subwoofer manufacturers would probably use even higher power amps.