Well let me add my voice to the chorus that Brian Ding is a terrific sub designer. While it's true that servo technology has been around for some time, Brian has spent six years tweaking and researching it in order to get better linearity, low distortion and deeper response than what one would normally have from a servo controlled sub. He has developed some propretary techniques to achieve these objectives which is why he suggests his technology DirectServo requires a special driver and a dedicated amp.
I've always had a keen appreciation for how a well designed sealed sub can perform and with that in mind I spent a week or so last summer conversing with him about the upcoming 15" version. He was very open about his ideas. From what I gather he started tweaking the servo sealed box 6 years ago with a 15" NCA driver. He preferred that driver because of its low inductance so that the zero phase crossover in the impedance curve was in the order of 150hz. Because that driver was expensive to make he later used a modified 12"TC Sounds driver which used a propretary short circuit ring in the servo mechanism to help achieve his design goals.
For the amp he likes using his own designed A370SE which is now shipped with a torrodial transformer, instead of the EI type. I don't know what else he has done to the amp except that it seems to match well with the enclosure and driver. The dedicated amp is critical to achieving low distortion levels in his subs. The extra SPL gain is mainly from high BL value and the larger enclosure.
While I do not suggest I know all the intricacies in his ideas or design, there is no doubt he has developed modifications which push the servo technology to the next level. It has been targeted at the DIY market but I believe its only a matter of time before he starts mass producing those subs. Judging by the many positive comments he has a strong following. It would be a mistake to dismiss the Rythmic subs as using old technology which he is calling his own.
