The idea behind the Stereo-Freq switch is that you can choose between two low-frequency cutoff frequencies, say 10Hz and 100Hz or mute the signal. Why would anyone want to do this? Late-night or early-morning listening. Deep bass travels easily through your house or apartment. By selecting the higher low-frequency cutoff, we can listen without bothering others.
But the real reason I made this new switch was that I needed a rumble filter with LP playback, as some LPs are horrifically warped.