There are not many subwoofers out there that have servos running them and that's a shame.
A servo is like a little microphone placed right on the woofer that listens to what the woofer is doing, compares that with what it should be doing and then makes corrections to get it perfect. Woofers are one of the few loudspeaker categories where you can employ realtime correction to get the output perfect. They are difficult to design and most subwoofer manufacturers don't bother.
Servos improve transient response, reduce or eliminate woofer overhang, distortion and flatten frequency response dramatically. Every subwoofer design I have ever been involved in was servo based and I would never use a subwoofer in my listening room that wasn't - especially since I need to integrate the bass with my very quick planar loudspeakers.
I am always amused when people tell me they can't integrate a subwoofer in their system because they always "hear" their subs as separate from their main systems. Of course that's true if you're not using a servo sub! A conventional subwoofer is like a slow moving slug.
How can you tell? Simply go to your woofer cone and tap on it. In a servo system the woofer will be as tight as a drum and resist your tapping - in a non-servo sub it'll "thud" and give way to your tapping. When you play the two subs, you get exactly the same results: one is lightning quick with transients, the other "wobbly" and "tubby".
Your system needs a subwoofer but not if you're going to add a slug. Add a race car instead.
