As far as I can tell, there is no mixer at the beginning of the signal path, only at the end. At the beginning you only get a Y option. I have cables plugged in and the mixer at the beginning of the path does not appear. I tried to put the volume block set to 0 after the Y then add the return block as the input, but it created a feedback loop...I've asked support and they seem to think there should be a mixer block at the beginning, just like at the end...but I can't get it to happen. I'm not trying to split Main L/R input, but trying to have the Main L/R on path A and the Return L/R on Path B. But any discrete input path would work for my needs....