I think you're misunderstanding or overthinking this. Each loudspeaker has two wires, a red and a black. Channels A and B go to the left, presumably front and back, and Channels C and D go to the right. Each loudspeaker connects to the terminals as marked. There's no difference in sound quality between the channels. This is a Karaoke amplifier, not a home theatre or HiFi amplifier.

S