Note what the reviewer said:
. . . With that said, the WAV files and high-bitrate MP3 songs we streamed sounded excellent.
and what the manual said:
. . . the unit is compatible with both WAV and FLAC when playing network audio.
Neither statement indicate that the WAVs were not
transcoded to MP3. One of the original design goals of DLNA was to hide the transcoding from the customer. With that in mind, many DLNA clients will support MP3 only, and will depend on the server to provide compatibility with other formats by transcoding to MP3. With this regard, JRiver is one of the best. I suspect the reviewer could not get FLAC to work because he was using his existing Microsoft software as a server, which is not nearly as good as JRiver.
If JRiver can transcode to 320 kbs MP3 without problems, that may be all you need. However, if you want to explore further, then you could try to get a copy of SR6006's DLNA certificate, which will tell you exactly what 'profiles' it supports. Also, the JRiver software should be able to supply you with logs specifying the results of the DLNA negotiations.
BTW: The main reason I prefer FLAC over WAV, even though the quality is equal, is that WAV has poor support for metadata, whereas FLAC has excellent metadata support. But it's only an issue if you actually make use of the data.