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.