Here's a graph of an AKG 414 LTD, omni.
The green, blue, & dotted black lines are all examples of bass roll-off. This graph shows how the microphone naturally rolls off at different frequencies.
The switch on your Church-Audio pre does this same thing, starting at whatever frequency it is designed to.
edit: to add that this is also known as a high-pass filter, or HPF. Different HPF's roll-off at different "steepness", so sometimes the cut is more gradual, and sometimes the cut is more severe.