How to increase headphone loudness beyond 100%?
If you have one of the supported phone models:

There's an app called voodoo sound.


Voodoo Sound in Google's Play store
Downside: Requires a custom kernel, only for supported phones (see description in market)


The paid app comes without a requirement for a custom kernel (but needs root)


Voodoo Sound Plus


Both replace the audio codec's driver and allow fine tuning and to turn on formerly unused chip features (see description).