To add to Haoleb's response and address one of the questions, I don't believe that a dedicated subwoofer would be powering the speakers from it's own internal amp. I think that the speakers would still be getting the power from the receiver or amp that was supplying the speaker-level signal to the sub.
Now, there are a number of systems that do power speakers from the subwoofer amp, but those are all-in-one systems like home theaters in a box (HTIB) or PC sound systems. Those wouldn't have speaker-level inputs though. I only mention this because your question didn't specify the types of inputs on the sub, so I'm not sure which you were asking about.