I like to comment on that with some insight how rooUPnP works.
In the advanced device setup of the connected UPnP streamers in Roon you always have to set “Use Flac compression” to “Yes”. This is the default behaviour so I did not mention it before (see screenshot below).
Why?
rooUPnP is configured to handle always and only Flac input from Roon. I did that to save bandwith on the Ethernet cable. Inside rooUPnP this Flac got decoded to either 16Bit or 24Bit PCM (bit perfect) according to what Roon sends to rooUPnP.
In the settings dialog in the rooUPnP extension for your streamer, you can define what is sent to your UPnP streamer.
According to this setting the decoded PCM got re-encoded by rooUPnP. Either to Flac, PCM or MP3. Re-encoding to Flac and PCM is lossless as far as you have selected a 24Bit output. If you select 16Bit output, 24Bit data will be truncated to 16 Bit.
Hope this helps a bit to understand the hidden functionality of rooUPnP.
So @Trickydickie: If your Naim NDS likes PCM more than Flac, please select PCM 24Bit in the rooUPnP extension for your NDS and switch “Use Flac compresson” in the “Device setup” dialog shown above to “Yes”.