I though it was 1)output 2)extension and 3) volume (as it pertains to LF analysis). I don't know why but it is hard to get deep bass and high efficiency out of a subwoofer. But the overall system sensitivity is usually at the mercy of the least efficient driver, and that's usually the LF or Midrange driver. Tweets are usually more efficient and you'll see L-pad. You can hornload a driver to increase its efficiency but thats rarely done. I think once you're in the 90db territory you're getting "warm" for high(er)-efficiency. 100's really freakin great and things above that just get obscene, like 113db 1w/m.
