How does the music know which wire it's on when it leaves the amplifier? Unless you have a crossover before the amp and are bi-amping the speakers, the same signal travels from the amp on both cables. Nothing changes until the signal passes through the terminals and into the crossover.
The biggest reason for bi-wiring is to remove the cheap jumpers included with many speakers. The second reason is using a preferred cable for high and low sections, as noted in the previous post. If you have only one cable, as in the OP's case, still using the jumpers, try attaching the positive lead to the high pass section and the negative lead to the low pass section. The reverse. One will sound better. This has many of the electrical benefits of bi-wiring without the cost. And buy better jumpers.