Put a 500K resistor in place of the Depth pot across lugs #1 & #3. Then connect a SPDT (On-On) toggle switch so one end goes to one side of the 500K resistor and the other end of the toggle goes to the other side of the 500K resistor. Then the middle toggle lug goes to lug #2 of where the Depth pot goes. In one position the toggle will be fully to one side or the other of that 500K resistor - sending the signal lug #3 of the Volume pot for the output.