ever been to a rock concert? Those subwoofers are larger than both Jake and the fatman put together. I doubt any subwoofing capabilities in a powerbook would make it " all that much better." Exploe the possibilities of line out, mesays.
The 12" powerbook has a third, hidden speaker. However, the two speakers in the 17" powerbook offer much better overall sound (probably due to large size and better separation. However, sound isn't exactly HiFi.... Plug in a pair of earphones if you want quality (or powered speakers).
I can't imagine any dedicated sub-woofer being part of a laptop or desktop. Granted, the newer ones are smaller than the older models, but there still has to be space for the speaker to resonate that deep base sounds.