It seems to be the natural response of the driver. I Have not had a pro calibration done, but do have my own tools to see the roll-off. I use the Dayton Omnimic. I guess its possible that the mic rolls off at 20hz but its supposed to be good to 5hz. I still notice quite a bit of deep subsonic stuff so it is clearly there. With the EQ, I was able to boost a bit below 20hz and also cut above 20 hz to smooth it out down below 20. With that much headroom its not a problem to boost below 20hz. Still cannot hit the drivers limits even with subsonic boost. I've tried not to do too much though for fear of clipping the amp.