I assume the common "side" signals should already only exist in the "Mid" portion.
Then, using the Bass Solo, I can then tweak my source material to get the bass phase aligned, or more defined.
I think the cut-off is just a lo-pass that rolls off the mid signal (so above this section is a "Mid Solo" button, and my use of this section is as if that button where pressed, and a lo-pass used to isolate just the bass).
Of course, the designer should have the ultimate answer