"Black Box" for Reflex Red Pickups

[Tunemeister]

Have a couple of sets of Reflex Reds, low impedance (I think around 300ohm) active pickups. I need a circuit design for the "black box" preamp that goes with them. Any ideas anyone? I have an expired "black box" but it is potted so I can't dismantle it to find out what is in it! Again any suggestions how to get into this without destroying the contents?