I have Plex setup on a Seagate NAS server streaming to my Galaxy S4 with the Plex client. I'm also using the S4 phone to bluetooth to an Apt-X receiver hooked to a regular stereo receiver system and speakers. Sequenced play is buggy, but I was having issues with that using UPnP Player, as well. I upgraded to Plex thinking that would solve it, but it hasn't. I don't think it's related to the phone or app, though. Anyway, mine is set for NO transcoding and all my files are FLAC. It has no issue playing them, so I have to assume the FLAC is coming through unaltered.
However, as others have mentioned - it will reset the Plex immediately if I attempt a FLAC high-resolution or SACD converted FLAC file. It seems only 16-bit FLAC files will work. That's still OK with me, because you're not talking very high quality DACs with either the phone or the bluetooth receiver, even if it is running Apt-X. I would think others would do just fine with mp3's and the like. However in my case, the music library is also the base for my directly-connected listening through high-quality DAC/amp/headphones.
Anyone have a clue about the buggy play? I've seen references to it with other UPnP/DLNA play, but they always seem to focus on some obscure phone brand with a less-than-compatible app. Mine will often get to the end of a song, then instead of sequencing to the next song in an album, it will jump back in the same song by about ten seconds and repeat playing the ending. Then it either hangs or cycles back to repeating the entire song over again (without the repeat play command activated). If I interrupt it by selecting the next song, then it continues on OK. Sometimes this happens, sometimes it doesn't. Any suggestions?