Speaking from experience, if you have any electronic dimmers in your house check to see if they are the culprit. Check for hiss with the dimmers off and on. doesn't seem to matter what proximity the are to your amps. I had a little buzz that only appears when the kitchen dimmer was set at halfway. Only way around it is to replace the dimmer or listen to my stereo after dinner...
Let us know if anything pans out. Good for everyone to learn...

Rob