The Deep-Hard switch in the photo shared by
@mavuser is super simple; just a passive high-pass switch, which sends the full signal direct to the volume pot in one position and in the other, sends the signal through a capacitor, effectively lobbing off that big boomy low end. This makes a lot more sense than the earlier "tone suck" switch, especially given the massive signal coming from those chrome humbuckers... even with the low end chopped off, it may well still give a bigger signal than your typical Fender bass pickup puts out. Also, when you cut those lows, it can really emphasize the mid frequencies and give you more defined punch (which I'm assuming is the "Hard"). This is actually a mod that I particularly like for blending pickups - my customized Rickenbacker bass has a vintage toaster in the neck position and a Hofner staple pickup in the middle position. Separately both have a nice big tone, but when blended they get a little muddy. I wired up a passive high-pass knob for the Hofner pickup, which allows for a very nice and clean blended tone between the two pickups.
Regarding the 60s / early-70s "tone suck" switch, I have a theory that it may have been designed with the amps of the times in mind? Bass speaker technology and cab design especially was not what it is today, not being able to produce and handle the big low end at higher volumes, so I could see the thinner sounds resulting from the "tone suck" switch allowing for more volume when using something like a vintage Fender Bassman stack. Otherwise, all that big low end produced by a neck-position Bisonic could eat up a lot of the power on tap without having a speaker cab that can properly create and push that signal, leaving us with lower headroom (earlier distortion) and less volume... just a though!
