... Come on, S&V guys, are you ever going to review a sub from Rythmik Audio? Their servo systems are just as spectacular as the subs from any other manufacturer. Possibly more so.
I have four of their L12 subs (the cheapest they make) in my home theater. I set them for optimal music performance and have found the system's headroom to be greater than what my ears and theater can take.
Total cost of the subwoofers with basic Pangea power cables, SVS sub isolation feet and Mogami Gold RCA cables was about $2,500. The performance of this system is very difficult to improve upon in my room unless you have no regard for your hearing.
One other great aspect that no non-servo sub can boast: the servo reduces distortion and any non-linear behavior by a factor of 3. It also means that break-in is basically unnecessary and that the subs will perform with incredible parity & consistency over their usable lives.
To the best of my knowledge, no other subwoofer can offer that (got this last bit from the guy who designs them).
As to SVS, why don't they include their Sub Isolation feet with their subs? They work really well. I'm basing this off of their photos.