A buffer stage has a gain of 1 (or sometimes a slight attenuation). In general such a low gain circuit, either a tube (cathode follower) or a transistor, has virtually no distortion... certainly way less than the associated gain stages. A buffer output stage prevents signal deterioration due to loading of gain stages that have high impedance output, so it is puzling that the manufacturer says not to use it. Perhaps he is catering to those who believe that ANY additional circuitry is always bad. These folk ignore the loading effect on the "naked" gain stage.