Servo is rare because it is a little more expensive to implement. Generally seen on higher end models such as the Paradigm Servo 15 (no longer in production) and a few Velodynes, as well as a few other manufactuerers.
One of the users here, Darien87, built a sub from them but I don't recall if it is the servo or non-servo. It should suffice to say that it sounds very good for what it cost to build.
I use a Rythmik amp for my Tempest and it does extremely well. I highly recommend these guys.
I'm actually very curious to hear the Rythmik/GR Research servo sub. The former GR sub already sounded great.