Do you have a source where it say's that the Xbox 360 doesn't allow lossless music?
http://support.microsoft.com/kb/906660
But uncompressed PCM (as in WAV files) is supported. I think that lossless WMA files are also supported (but who cares)
FLAC and APE formats are definitely not supported
I'm still really curious for the need of external dac - Cambridge Audio DacMagic
A) As I said before - you have excellent sound card - I'd trust it and output the audio analog to your stereo receiver - it's extremely capable
B) Your Outlaw receiver has built-in capability to play straight from pc by usb cable
C) Optical spd/if from motherboard to dac is also possible, again without any quality loss - but I'm not sure why?
Any option will provide same excellent sound quality, but I'd go with option A since analog run are not limited to 15ft like USB is
Now to your main question - why streaming uncompressed WAV to Xbox 360 using Windows Media Center end by "possibly" being compressed - we'd probably never know since Microsoft is not going to tell us