I would like to introduce you my headphone station project. Nothing great, but it does everything I need and that very good.
I mostly listen to my music through headphones. However, since most music is mixed for speakers, it often sounds unnatural, because a strict channel separation can be heard via headphones. This is different with loudspeakers, because reflections from one channel arrive at the other ear quieter and
with a time delay. On my search for remedy, I found the freely accessible passive Crossfeed circuit by Jan Meier, which is adaptable to the source and the amplifier. http://web.archive.org/web/20090521143201/http://www.meier-audio.homepage.t-online.de/%20passive%20filter.htm
The reflections with time delay of the sound from loudspeakers will now be simulated for headphones through the crossfeed circuit. It sounds now more natural and hearing becomes much less stressful.
My setup consists of RasPi 3 with HiFiBerry DAC +, Volumio 2.361 and activated GIPIO Buttons plugin for the shutdown function. For visual reasons the button is on the back side of the case.
On the Front are 2 rotary switches to adjust the crossfeed to source and amplifier and a third for the strength of the crossfeed effect. The toggle switch on the left, is a bypass switch for the crossfeed. There is also a (blue illuminated) power off switch on the right, to de-energize the RasPi. As headphone amplifier, I have a Schiit Magni 2, that has enough power for almost every headphone.
Finally, I would like to show you the setup at work, of course. Incidentally, the lettering on the front panel is laser-engraved. 
