This innovative dsPIC30F2020-based project showcases an interesting approach for reducing speaker distortion and increasing bass response. A dual-coil speaker driver is implemented in the design, but only one of the coils is connected to the power amplifier. The other coil is used for electromotive feedback.
This is not a new idea, but many do not implement it due to the multitude of factors that have to be counted for the analog signal.
Digital Subwoofer Amplifier with Electromotive Feedback Reduces Distortions: