This is from JDS Labs https://www.jdslabs.com/faq
It all depends on your source
To choose gain for Objective2, refer to your audio player's output strength:
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) →...