It gets even more confusing when you factor in electronic crossovers. If you have a normal HT setup, the bottom two octaves are filtered prior to amplification for the 5 or 7 speakers. The sub fills in the 20-80Hz range, using it's own amplifier. Thus, your 100 watt amp - unencumbered with reproducing the power-hungry frequencies of 20-80Hz - can now play louder and cleaner. This is one of several reasons to buy a quality powered sub - especially if you use a reciever to power all 5 or 7 channels.
