Commercial subwoofers are not only a poor value, they are POOR poorformers in comparison to what DIY projects can obtain. I apologize but I'm too busy to rehash physics and why this is so at the moment. As far as close loop servo.. No, it is not that simple either. Read this thread from page 3 onward and see why DIY projects VASTLY in MAGNITUDES OF ORDER outperform $30,000 Krell MRS subs or any commercial subwoofer. For instance, My Black Hole subs outperform (2) $18,500 = $36,000 Wilson XS subwoofers due to the superior construction, enclosure, and superior physics of the design... IF you know the physics, then you know what you are talking about. Subwoofer physicss and design is fairly easy to learn if you invest the time, know what books to read, and which people to consult/participate with... Just read this thread from page 3 onward for a general idea on the best bass available bar none...
http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=299681&perpage=20&pagenumber=3

7.1 System