Does Plex playback 96/24 FLAC or ALAC?
It's not plex you need to worry about, it's the receiver.
A little background on how UPNP servers work, two methods: The first method, and the one you want, simply streams the files as is. UPNP does not stream in any special way of it's own, it simply makes the music files available to other devices on the network. This will give you 100% the quality of the file when it reaches the endpoint. The second method, used only when method one fails, UPNP (some) software (including plex) can transcode files from their original format into another one if the endpoint cannot play the original format. For instance if your receiver cannot play 96/24 FLAC plex could transcode the files to mp3 before sending them off.
You would probably be best off getting a streaming device though. Something like a WD TV would be able to see files shared on the network, eliminating the need for you to run any special software at all. The squeezebox's are nice, but I think they're horrifically overpriced. To clarify DLNA is synonymous with UPNP, the A/V manufacturers thought that UPNP didn't have a nice enough ring to it so they renamed it. As jcunwired said there are solutions to FLAC in WMP, but I'm not sure myself if they will add UPNP server support for the FLAC files, or simply playback. Additionally I rather dislike WMP as a player and server, so I would still suggest plex if you go that rout.