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 set mine to 94 on a serranoltex device (SAMSUNG S4 Mini INT) and it’s too loud for some of my MP3 at max volume, still great for most of my others.
Ouch loud :)