effect should give you the feeling that you stands in a real room with
  a complete 5.1 speaker set when using only a headphone, providing a
  more realistic sound experience. It should also be more comfortable
  and less tiring when listening to music for long periods of time.It
  works with any source format from mono to 5.1.

  -> please try it and feel free to give me some feedback. Some
     improvements are already planned (adding echo, more configuration
     options, ...).

NB: whereas the filter itself is in a (very first) stable version, the
    way it is integrated to the filter chain is only a _temporary_ hack
    since it's the audio ouput core (input.c actually) which is directly
    responsible for it. Integrating it in a more suitable way will
    probably require some work on the way the filters are selected as
    well as on the configuration level, but I'm working on it :)