Less resistance means less energy lost to heat, which means more efficiency electrically. The standard measure of efficiency in audio terms is the 1W at 1 meter dB level. In other words, a microphone is set up 1 meter away from the front of a speaker -- usually a point somewhere between the tweeter and midrange or woofer. Then the speaker reproduces a signal with exactly one watt of power. The resulting sound is measured. More efficient speakers will be louder.