@Zenvota @Drivenby Plex is now working fine. The only change I made (as far as I can see) is to set the audio device to default instead of purposely choosing Mobius. A big thank you for all your help!
They're working really well - dialogue is firmly placed in the center, soundstage is nice and wide, rear effects often come from behind me. If I was going to nitpick (and it's ludicrous) I'd say that sound from the front is not "in front" enough but that's getting really finicky. The sound overall is really nicely detailed without being too sharp. I have engaged bass boost but may play with that a little to get a bit more.
Overall, I'm very pleased. I'll put together a proper review for here when I've finished the paper I'm writing (or supposed to be writing).
I'll add a spoiler here though - I have no interest in bluetooth headphones, I would always go cabled instead, I had no intention of using the bluetooth on these. Hooked them up to my LG V30 just out of curiosity, set it to Best Audio Quality (thanks for that LG) and...wow....serious wow. They're excellent.