In that case you might have to use sink.primary; this does handle the output to the 3.5mm socket as well as the main speaker, I'm not sure if there's an individual sink that only goes to the headphones (if there is that'd be a better option). If you do go down this route I'd recommend being careful, and being very aware that any damage will be outside warranty .
pactl set-sink-volume sink.primary 1.5