Time for bleeding ears:
– Mount /etc read/writeable with your favourite file explorer (Root Explorer can do it for example)
– Locate /etc/mixer_paths.xml
– Open it with your favourite text editor.
– Play with the values for „RX1 Digital Volume“ and „RX2 Digital Volume“ in the „Headphones“ path
– Save
– Reboot
– Check the results
Be careful, it can get very loud and noisy if you push it too far. I guess it might even damage your headphones/ears, so start your tests with a low volume. I set mine to 93 on my bullhead device (Nexus 5x) and it’s too loud for some of my MP3 at max volume, still great for most of my others.