Thanks for the suggestion. I installed the apk and made some discoveries.
LDAC switches off if you use the equalizer in the Sony Headphone App. It lets you know when you try to adjust it. Goes right back to the primeval SBC (earlier than the first Aptx codec). So I went looking for the equalizer in the Fiio Music app. In case anyone is baffled like me, you have to tap the cover art in the Now Playing menu, then you can adjust the EQ. The FiiO one sounds fine and seeing as its the only one I can use, i'm happy.
Also, LDAC at its highest quality level sounded great but due to being a 2.4GHz technology, it fought with my 2.4GHz home WiFi and lost, stuttering. I turned the M6' WiFi off, and LDAC highest quality mode stuttering went away. Aptx HD on the same 'phones was not similarly affected. And sounds pretty damn good anyway. (All listening was with FLAC files).
Really loving the player, now that I know what it can do.