Can someone please give an explanation for how the two line outs (outputs 3 and 4) on the 26c work? Outputs 1 and 2 are controlled by the knob on the front panel. What controls outputs 3 & 4? Can 3 and 4 be used for a second set of powered studio monitors so that a mix can be compared between monitors on outputs 1 & 2 and outputs 3 & 4? I just don't understand what the two outputs (3 & 4) do. Thanks in advance for any and all responses!