Any time the impedance drops, the wattage an amp is capable of producing increases. We can thank Ohm's law, which can be stated as volts = watts * impedance. With the voltage fixed, lowing the impedance will increase the wattage. In theory, this means that using a lower impedance speaker can wring more power out of the same amp. The only catch there is that some amps' power supplies can't keep up with the current demands as the wattage demand increases under a low impedance load. Here is a link that my web search turned up that might be of interest to you.