Instrument cables would be the correct choice, speaker cables carry the amplified signal from a power amp to the speakers or crossover. If you were trying to cover more than 30 ft, I'd suggest any balanced cable, TRS or XLR, but for 3-6 ft that you're likely using, there should not be any real difference, provided the BX's are wired to accept unbalanced cabling. Worst case, you'll lose 6dB, and have to turn up a little more.