One knob, overall gain essentially determines the output signal strength of the bass. The Slap depth adjusts the EQ of the slap switch (The mini toggle on the front). It doesn't have an effect when that switch is off. Its basically to balance the volume between the two settings. If you play metal bass ,you'll probably never have the slap switch on, since it essentially is a giant mid scoop. I'm not even sure what mine are set to. You should have enough EQ range on the front of the bass to make up for it, assuming you're not flipping between the two modes constantly.
Sir! I have the Curbow again. It has those gay little rotary dials you turn with a screwdriver in the back one for 'Slap Depth', and the other for something else. How best to set those for a generic metal setting? It'll be played picked.