What he said. The upper ring adds a boost at the cut of frequency of the filter which is set by the lower ring.
The lower ring does not add bass as you turn it ACW it reduces the higher frequencies the bass is always present it is the higher frequencies that you either let through the filter or don't. To increase the baas the upper ring will boost which ever frequency you select with the lower ring. So if the lower ring is fully anti clockwise you can boost the area between 70 and 100 Hz.