Further to what Dave said, when a design sums the left and right inputs, the attenuated L&R signals are often fed through opamps in a differential scheme before summing. This is not for signal quality but rather to prevent problems when connecting amplifiers which have BTL outputs (Bridge-Tied-Load or "bridged"). The L&R "-" speaker outputs on these amps are not a common electrical point, to short them together would severely damage the amplifier. While very uncommon, amplifiers (receivers) with this output configuration do exist. Mostly inexpensive models.