These are the gain setting recommendations from JDS Labs, this should give you an idea of which setting to choose . So unless you're running from a phone, laptop or tablet choose the medium setting
1.0VRMS (iPod, iPhone, many Androids, typical laptops, etc.) → Choose gain of 2.5/6.5x
1.5VRMS (most iMacs) → Choose gain of 1.0/4.5x
2.0VRMS (OL DAC, most aftermarket DACs) → Choose gain of 1.0/3.5x
2.1VRMS (ODAC RevB) → Choose gain of 1.0/3.3x
>2.1VRMS (professional audio equipment) → Choose gain of 1.0/2.5x