Another unique design for your considerations. An ingenious fellow by the name of Thomas Danley has employed a Direct coupled DC motor to drive a subwoofer many years ago. Differently than Eminent-tech for certain, but never-the-less very effective. His patent was bought out by Servo-Drive I believe. Check out www.servodrive.com to see the current pro and home versions. Also interesting was the project, from which the design for the Servo-Drive subs evolved from. The original Tom Danley designed sub, the Pachyderm, was created to reproduce subsonic bass below human audibility. The Pachyderm http://www.ultimateavmag.com/features/704way/index3.html. I have "experienced" the Contrabass, and folks it just don’t get much better - :-) in my humble opinion. The real ingenious part of this design to me, is how efficient it is, and the fact that the woofer becomes more linear the harder(louder) you play it – the opposite of conventional cone subwoofers.