Speaker configuration: "Use Windows Default" or "Pro Logic"?

[Malangyar]

Hey guys, does anyone know what Pro Logic does when choosing speaker configuration? Is it virtual surround? I use headphones and just want the best sound quality available. Currently I've set it for Pro Logic but not exactly sure what it does. Does it upmix or downmix the sound, or both? I've tried searching google but it just makes me more confused.

Thanks for any help!