Paul, your explaination on usage of the KC300 is excellent. Except in case 1, the L&R outputs from the keyboard should go to Channel 2 or 3 of the KC300. Only Channel 2 or 3 support stereo inputs. Channel 1 & 4 are mono.

arnothijssen, KC300 also has a stereo mixer (channels 2 & 3). Each channel accepts L & R inputs.

Henry