I have a synth connected to the front input 1 connector. The switch is set to 'line'. When I press a key on my midi controller I can see activity on the input 1 led but no audio comes out of my main stereo output. I have another synth connected to inputs 3 & 4 and this works fine, I get audio in my main stereo out. Is there something else I have to set for the 1 & 2 inputs to work ?