My mistake. When you indicated 120-dB, I thought you were referring to audio volume levels instead of electrical watts.
It's not really your fault.
Decibels are, by definition, a ratio scale. "120 decibels" isn't an absolute quantity, it's a statement that X is a trillion times larger than Y. For sound, the reference volume (0db) is a sound pressure of 20 micropascals, but nobody actually knows that, so confusion reigns supreme.
Also, there's no difference between electrical Watts and and the work-energy in a sound wave. It's all Joules-per-second in the long run. On that score, you're right that I assumed a speaker would convert eletrical power to sound pressure with 100% efficiency, which is utterly wrong and highly dependent on the speaker. My bad there.
To make things even more complicated, we have to take the inverse square law into account. Sound pressure drops with the square of the distance from the source, so a truly complete description would be something like: "120db above 20uPa at a distance of X meters from the source." An iPod's earbuds have a rated maximum output of 103db (some people claim they can reach 130db), but they're like, 1/2" away from your eardrum.
Touching back to the original specs, I still think 120db is massive overkill. The audio pain threshold is around 100db, which represents a hundred times less energy. 120db is into "momentary exposure can cause permanent hearing loss" territory.
When you void a product warrany, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.