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 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.
What sub is this? Why do you have it hooked to your amp if it is powered. Try a line level hook-up to the sub from your pre-amp and see if that helps.

Some subs are designed to hook up this way ( REL, Art of Sound and a few others) and their amps have separate circuits which accept LFE and high level Speakon cables simultaneously. If the manufacturer doesn't recommend this type of connection, try it Dave's way or maybe R/L pre-outs.
It is a Infinity Intermezzo 1.2 subwoofer and the preamp, has just one pre output. Even if I can get a Y splitter, what about the timing issue between the preamp signal that goes to the amp and then to the horn loaded speaker and that which travels directly to the subwoofer amp section? There are many people out there that complain about this subtle point, is it that subtle that is innaudible or is it noticeable?