@James my question is now finally answered 
The use of the gain switch ties into the volume control used on the unit. Most headphone amplifiers feature an analogue volume control (unlike DACs, which tend to feature digital volume control systems). One important factor that must be considered when designing an analogue volume control system is the range of operation through which the potentiometer used to control the signal provides good matching of the left and right channels.
At the edges of this range–when the volume dial is set towards the minimum or maximum position, for example–the volumes of the left and right channels may not be equal. This creates a number of problems, such as skewing the soundstage. For this reason, it’s always best to use analogue volume controls within a sensible region, close to the 12 o’clock position if possible.
Higher quality potentiometers offer–among other things–a greater range of operation where the left and right channels are well-matched. With the Lina Headphone Amplifier, the volume control potentiometer sits in the feedback loop of the amplifier, and not in the signal chain. This means fewer components in the signal chain (which is crucial to achieving good performance). It also provides a major benefit when using the amplifier with sensitive headphones: when the volume control is turned down, the noise floor of the amplifier goes down with it. This is important as higher noise floors are readily audible when listening through sensitive headphones and IEMs.
The potentiometer is trimmed in-house at the dCS factory to achieve a level of performance above what its specifications would suggest. This, paired with the flexible inputs and gain switch, means the Lina Headphone Amplifier is flexible enough to be paired with almost all available source devices and headphones.