Just a couple of facts before I carry on.
You asked for help setting up your system less than five years ago, not ten. It was June 2014.
You were advised by Bob, one of the developers, and in that discussion you said:
Thanks Bob. This is great news. Just to be sure I understand the setting 100%. Is it: Tools, options, media network, configure DLNA servers, audio mode: ORIGINAL.
So you clearly understood what settings were required. As you have now clarified that you were using the "Generic DLNA" configuration until now, and you have deleted it, we will never know if you implemented that change. So we will never know if you have really been listening to mp3s for five years or not. There was no need to delete the Generic DLNA, particularly before you got everything sorted out. But what is done is done, unless you want to Restore a MC Library Backup and have a look at what you had in those settings. But if you are going to do that, do a manual Library Backup first, so that you can Restore to your current condition easily after having a look at what you had before.
In all likelihood, you have been listening to your high-res music in their original format, and not in mp3 format. But something changed along the way, so that you were sending mp3s to the ND5 when you set it up.
By the way, when you play "direct" from you Synology NAS to you ND5 (or previously to your Linn) streamer, as described here:
I just bought a Naim ND5 XS2 streamer which displays the type of file it plays. <snip> When I use the Naim app on my Ipad to choose the exact same file on the NAS, the display says it is playing a flac 16-bit file.
There is no doubt that what you were actually using as the transport mechanism was DLNA, which means that the Naim App was acting as a DLNA Controller, the Naim ND5 XS2 was acting as a DLNA Renderer, and therefore your Synology NAS must be running a DLNA Server. In which case, the DLNA server could have been doing anything to your high-res music before it sent the audio to the ND5 (or the Linn previously), and you would have had no idea.
You should really shut down the DLNA Server on the Synology to avoid confusion, and just use MC as the only DLNA Server on your network.Now just a little correction:
1. You set the format Mode to "Specified output format" and the mode Format to "PCM 24-bit" as you suggest. Then setting the DNLA format don't really matter. This will As you say it is always sending PCM. The only con of this option is that the controller Renderer will always display 24-bit even if the file is a 16-bit or MP3.
As to the issue of playback not working at all now:
I have properly changed too many parameters trying to fix things myself so now I can’t even play MP3 anymore! I get this message when trying to play back: There was a problem controlling the selected DLNA device (both ND5 and Klimax).
I think Brian's suggestion is a good one; Reboot all the equipment. DLNA devices can get stuck, stop communicating, and generally forget their status at times. You have been changing a lot, so a reboot all around is a good idea.
If that does not solve the playback problems, my first thought would be that the Renderer can't play the format you are sending it. But the ND5 seems to be pretty capable. So test again and report back on that issue.
EDIT: It sounds like the restart/reboot has fixed that issue.
PS: Thanks for sharing the screenprints. That helped us all understand what you had done, and what you hadn't, a lot.