S "switch Jack", as has been pointed out already. The Idea about the IC is no good... that device only mutes the speakers when headphones are plugged in, albeit without a switch jack... it was apparent that he didn't understand your Post. Use the switch jack to switch off the audio to the power amp when the line output is in use. If you place the jack before the Vol control then your line out won't be affected by the volume setting which is normal for a line out. In fact that's why it's called "Line Out".
Doc