The majority of woofers, in the majority of speakers, send information, back into the amplifier, called EMF. Many articles on the subject. Some manufactures believe in the concept, some do not. I have always been an advocate of biwiring, and, passive biamping, when the speaker was designed in this way. Passive biamping takes the " back EMF " situation, a step further. The amplifiers, in this case, have less work to do, as they no longer need to reproduce the full bandwidth of frequencies.