<font color='#CCCCCC'>The lower the frequency of sound, the greater the amount of air that must be moved, and consequently the greater the amount of power needed to move that amount of air. For example, it take four times the amount of air to produce sound at 20-40 HZ as it does to produce sound at 40 to 80 Hz. So, because of the high power requirements at low frequencies, it really makes sense to power a subwoofer with a separate amplifier, thereby taking that load off of the other amp that powers the mids and highs.
However, to a lesser degree, it also makes sense to tailor the power and resistance of an amplifier to a specific speaker. I know Paradigm sold active speakers for a while and then dropped them. I would guess that cost has something to do with the scarcity of active speakers, as well as the fact that the owner of a receiver, or the owner of separates with amplifiers, would be paying again for something he already has.</font>