Just because you can play the file does not necessarily mean that it isn't being trans-coded to another format before it gets to your headphones.
Great idea, I learned much in this thread myself.
Regarding your comment about files being trans-coded, this definitely is an issue on Windows as the Windows mixer will usually intercept audio playback and muck about with it. I know of two ways to avoid this, either set you AVR to bitstream (ie. no A/D is done, just send the digital stream) or better yet use a great media player such as JRiver Media Center 17. It can (painlessly) use WASAPi - Event Style to bypass the Windows mixer completely and let the output device pull data as required instead of Windows just feeding data dumbly.
If you are going to do any testing on Windows for music playback, JRiver Media Center 17 (18 coming soon) is to my knowledge the best player out there.