What is a "servo" sub and any advantages?

For the ultimate in sub's check out Tom Danley's ServoDrive ContraBass subwoofer system.
http://www.mindspring.com/~sdinc/pages/contra.html
Talk about slam & dyamics! This thing is definitely major-league! The woofers have no voicecoils per se; they incorporate a stepper servomotor into a dual linear driver combined with two passive radiators. The technology is also used in commercial theater applications. The ContraBass will absolutely rock your world!

Isn't "error correction" aka a servo on a speaker similar to trying to stop spilling the milk after it's already spilled or in the process of being spilled ? Sean
>

Servo's respond to an error that has already occured. Then it corrects this error according to the designers preference. This is in and of itself not a bad thing. However, it does create distortions of its own during the correction process that may or may not be important to the listner.
I personally consider servo subs good for home theater but not for music.