Just installed Airport Express and have it via Ethernet, piped into my 509. I didn't do as much research on purchasing the 509 as I should have, otherwise would have probably gone with the Pioneer or Denon receivers which have airport express already built in. All my music/mp3's that I've ripped to my desktop have used the lossless Apple codec in the pass 8 years. While I was able to stream some mp3's via WMP to the 509, it was at a lower bitrate.
I got the idea from this thread of buying the Airport Express and using that to solve my dilemma of converting my entire music collection back to a codec which WMP could play or re-ripping all my cd's. While not the least expensive way to do, it turned out being the easiest way for me to listen via iTunes. Everything just works! The only downfall, unfortunately, is the display panel doesn't show the song playing via AE as it did when streaming from WMP.
I gotta say that the sound coming from the speakers does sound excellent via toslink!
Thought I'd add my experience for a remedy for what could have been a regretful purchase.