A friend is putting together a hi fidelity system using a servo drive subwoofer. After hearing the advantages, I became curious as to why they aren't common in pro audio. Obviously, they're going to be more expensive, but for people willing to spend, they promise tighter, less distorted bass. From what I've read, servo controlled subs have gotten better in recent years.
Some active subwoofers use a servo feedback mechanism based on cone movement which modifies the signal sent to the voice coil. The servo feedback signal is derived from a comparison of the input signal to the amplifier versus the actual motion of the cone. The usual source of the feedback signal is a few turns of voice coil attached to the cone or a microchip-based accelerometer placed on the cone itself. An advantage of a well-implemented servo subwoofer design is reduced distortion making smaller enclosure sizes possible. The primary disadvantages are cost and complexity.