What is a "servo" sub and any advantages?


I am considering a purchase of a used Infinity Servo Sub, but don't know what the advantages/disadvantages are of servos. It doesn't seem that Infinity has any in their new model lineup, I know of Paradigm's Servo 15. Should I be cautious about purchasing this?
I own a Servo 15 and love it. The servo is control mechanism that returns the driver cone back to its original position faster than the magnet will. So the advantage is that when the cone is traveling to the furthest reaches of its excursion it sometimes cannot re-act fast enough to respond to the next beat so the servo kicks in and takes control. Also the servo prevents the cone from over re-acting and producing distortion. Some say that the servo can restrict the sub from producing a true and accurate tone but I fail to believ that the human ear would distinguish the difference if any
For the ultimate in sub's check out Tom Danley's ServoDrive ContraBass subwoofer system.
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.