subwoofer+speaker+same binding=low level hiss

I have connected to the same binding post, a fullrange, hi sensivity horn loaded speaker and also a powered subwoofer. They run in parallel, with the purpose of seamlessly integrating the timing of both the highs and the lows. Nevertheless, when I get near my horn loudspeakers, eventhought the main amp is not powered, I hear a low level eletronic hiss. When I disconnected the speaker cables that ran to the subwoofer, the horns did not produce any noise. The subwoofer, when I stick my ears next to it, do not make any noise either...it has a standby amp feature.
So, I wonder what is making the noise...can it be the cable that runs to the sub picking up spurious noises from nearby TV sets and other devices? How can I solve this? the speaker cables used to connect the amp to the subwoofer are audioquest, with heavy spades....Maybe, if I switch it to a different cable, that is more shielded or thinner, I can avoid this? Any experiences with this will be appreciated.

PS: my system was put together to avoid any sort of noise....the amp is battery powered, so is the preamp...therefore, this noise, even if it is innaudible, lest I get close to the main speaker defeats the purpose of the set up.