Cone Control Systems for Subwoofers

On the Secrets of Home Theater and High Fidelity website, they have a review of the Velodyne CT-150 subwoofer, which says the following:

"Velodyne's top of the line subs use a solid state accelerometer mounted on the voice coil. Current sensing feedback is less accurate than servos, but it is also less expensive. Current sensing feedback works by knowing what the current is that is being fed to the voice coil by the amplifier, and sensing what the current is that is being generated by the actual voice coil movement within the gap of the magnet. The difference in the waveforms between the amplifier feed and the voice coil generated current (this difference represents distortion) is inverted and fed back into the amplifier. This cancels out much of the distortion produced by inaccurate movement of the driver cone. The CT-150 is the only member of the CT series that has current sensing feedback. "

How do you suppose one could implement a system like this?