Thanks for posting. Another way to think of this is the upper frequency controls the shelf (boosting anything below that corner freq) and the lower frequency controls the highpass filter (rolling off the lows below that corner freq). The high pass filter takes out the sub lows and rumble which muddy up the sound and stress the speakers. Increasing the lower setting from 20 to 40 or even 60 for less deep bass would tighten up the bottom. Then increasing the shelf up from 40 to 80 or 100 adds more meat to that all-important first overtone, making the upper bass more solid. I'd be wary of moving that shelf too far into the low mids or you might be swapping rumble for boom. This is a very cool feature for an onboard preamp and might help me at one place I play that takes bass direct to FOH and runs the subs too hot.