Older Infinity subwoofers--what is a servo sub?

Have been looking at used subs and have the opportunity to purchase two Infinity models--

the RS 10 (from the Reference series)

10" Servo Subwoofer

Infinity says they are both basically good products, but since servo subs are not in the mainstream of what I've seen I have questions? Anyone have experience or knowledge of the older Infinity stuff?
Servo subs are now more in the mainstream than they used to be and Infinity (Arnie Nudell, actually) was a pioneer in this. Basically, the servo sub has a device (accelerometer or sensing coil) that feeds info back to the amp/controller so that errors in performance are correctable. That's a simplification and there are many paradigms for this correction.

OTOH, I have no first hand experience with either of these models.
I've got an RS Subwoofer, 10" driver 100 watts and can find no info about it. If anybody has specs original price let me know. Thanks
I suggest you post this as a new thread with the model number if you hope to get a useful response.
Servo circuits are basically an outgrowth of the negative feedback circuit that takes into consideration the actual movement of the driver(s) itself. If the driver is "throwing" too far, the amp cuts back signal to correct this. If the driver is not "throwing" enough, the amp sends more signal. As such, the amp is trying to correct for non-linearities in both the signal and the driver(s) / cabling being used.

The only problem with this is that the servo system can only try to correct a problem after it starts to occur. As such, servo's are always playing "catch up" and can't correct errors before they happen. Having said that, it is somewhat true that combining an active servo-amp with active equalization can give a speaker system a form of "psychic ability" i.e. ability to correct a problem before it occurs. This is due to the fact that the engineers know how the driver / amplifier combo is going to react in a given situation and can program in corrective EQ to balance that out. This is one of the theoretically "great" things about active speakers i.e. the reproduction / amplification chain were all designed to work as one unit. Obviously, room acoustics and speaker placement do vary, so good engineers will give you some adjustments to work with in any servo or active speaker type of installation. Sean
Subwoofers such as Velodyne and the Genesis 928, (Nudell is associated with Genesis) use accelerometers. I find that there seems to be less boominess. Lower distortion measurements indicate they seem to work pretty well.