how to wire up potentiometer to subwoofer
Some audio taper potentiometer that you bought at Radio Shack is unlikely to be useful the way you imagine unless this "subwoofer" is really a lot more than just a subwoofer.  The impedance of the signal that drives a speaker, like a subwoofer, is very low, well under 8 &Omega; assuming a 8 &Omega; speaker.  Pots for volume control are much higher impedance, and can't handle that kind of power level anyway.

If this "subwoofer" really isn't, and actually contains a amplifier, then the pot might do what you want.  You can tell this case several ways.  If there are only two wires going to this unit coming straight from a power amp, then the pot won't work.  For the pot to have any chance of doing what you want there has to be separate power, like a line cord, connected to this box.  In that case the signal could be "line level" and is amplified as needed by the speaker inside the box.

If you have the latter case, then it should be pretty easy to figure out which wires are what.  Turn on the unit and put your finger on each line.  When you touch the audio line, the hum picked up by your body should be amplified and audible in the speaker.  Touching the ground line won't have that effect, and might even make any slight hum coming from the speaker weaker.  Connect the ground wires as usual, but break the audio wire connection.  Connect one end of the pot to the common ground, the other end to the audio line coming from the preamp, and the wiper to the audio line going into the speaker box.

Can't you just turn down the bass at the preamp so all this isn't needed?