You guys are right. I looked at my HC in the Audyssey app and the graph seems to end at 20Hz. However, @desray Audyssey Guide says that Audyssey flattens the curve down to 10Hz, so the fact that I have such a high boost at 20Hz, should mean that there is some effect below 20Hz.
The other issue I see is that the bass shaker doesn’t actually produce sound. It just transfers the vibrations to the frame of the chair. So such vibrations aren’t actually being picked up by the mic, unlike the subsonic frequencies produced by the subwoofer. So it will be trial and error. How do you guys actually do it with the minidsp?
Another option is the Audyssey MultiEQ-X desktop software. That allows you to adjust the Audyssey curve down to 10Hz 
MulitEQ-X is a neater solution than the minidsp, but since it costs money, might be better to buy some hardware and then the solution is portable to say Dirac. Although its unclear to me how Dirac in a Denon amp would interact with Dirac in the minidsp and how Dirac Bass Management and Direct ART would integrate with two Dirac devices run together…