the shhhhh will simply be static or white noise, normally you should only get something like that at really high volume setting....however if its happening at low volumes too you may well have damaged something....
the other possibilites could be you are running one of the cables too close to something it shouldnt be run close too....and its getting interference.....
i remember when i bought my REL Quake subwoofer, its the baby in REL's very impressive lineup (still set me back £350..lol) it came with a crappy low level phono cable for the LFE channel.....and when i hooked it up to my surround amp, even on a low setting the hum was unbearable, the cable wasnt just not shielded, it was thin as the wire itself, a bare 0.5mm of plastic coating...lol
So i went back to ask, and they said thats all you get with it, with their more expensive models (the £700 Strata5 i have now) you get a decent hi-level neutrik speakon connection, but u still gotta buy a low level phono...lol.....ended up forking uot £40 for a 5metre Qudos....nice tho, triple shielded, no hum, even at loud levels when no sound coming thru the system