Monitor hum

[Smenelian]

I just got some new monitors and had a problem with some kind of electrical interference, which I fixed by using the DC in on my firewire interface instead of using firewire bus power through the normal 6 pin cable. I tested the monitors in the store through the same interface on the same firewire cable that was giving me problems, and there was no hum in the store. Why might I have a problem when I set them up in my home studio? If I can go back to the other firewire cable, I'd prefer that so I can have a free outlet on my power strip.

Also, I'm using instrument cable to run the audio through the monitors at the moment. Do I need to get some speaker cable or can I leave it as it is?