You can do a Jugaaar..
1) Unscrew/Open your subwoofer.
2) Locate the wires through which the subwoofer is connected to the circuit, Cut them off
3) Now connect external wires to the cutted wire and drag them out of subwoofer, now close back your subwoofer box.
4) Connect Your subwoofer via External Amplifier based on Subwoofer's RMS rating
5) Connect it to RCA Output of your PRE
6) If your PRE can handle it directly without amplifier then bridge two speakers output wires in to one and connect it to subwoofer.
Do tell me if you really worked out on this plan.