Why does SoundID Reference lower my output volume?
By default, the output is lowered when the Safe Headroom feature is enabled. SoundID Reference cuts and boosts certain frequencies to compensate for the inconsistencies in a particular output's frequency response. Boosting certain frequencies creates a risk that the digital audio will peak above 0 dB and clip. The Safe Headroom feature lowers the output volume by the maximum peak in the correction curve to prevent the output from clipping (prevents gain-staging related audio distortion).
You can disable it at the risk of introducing clipping by turning off the Safe Headroom feature and turning the volume fader all the way up.