The board has only one input, so if you plan to use a normal stereo feed supplying a single P48 board, you'll need to sum the two stereo outputs. This is easily accomplished by using a pair of resistors - the value should be between 2.2k and 4.7k. If this is done, replace R1 with either a 100 ohm resistor or a wire link.
VR1 is used to change the gain of the second integrator. The level through the controller can be set to make sure that there is no distortion - there can be a huge amount of gain at low frequencies, and if the gain is too high, distortion is assured!
The high-pass filter is designed as a peaking type, and gives a response that is almost perfect down to 20Hz. The lowest frequency can be tailored by changing C1, C2, C3 and C4. As shown, the response peaks at 18Hz, but you can use 68nF to increase this to 27Hz, or 47nF for 39Hz. See Table 1 for the full range of values.