Please forgive my ignorance but now I am totally confused.
So the dac is currently being fed PCM (not sure what that is). Is this better for the dac than being fed WAV and if not is there a way to set it up so the Naim dac is fed WAV (which as you say is meant to be the best thing to feed it).
PCM is what DACs understand and is what all digital audio starts as and ends up as at the DAC stage of any audio system. Both WAV and Flac contain PCM data they are file formats that contain PCM data. Flac uses a lossless compression system much like a zip file does to make that PCM into smaller files. The process of decoding these back to raw PCM has a slight overhead and on older DACs that have less processing power it may have a very minor affect on the sq. This is why Naim recommend WAV when sending via their streamer as the processing to raw PCM to the DAC stage uses less processing power. If you not using a Naim streamer it’s moot and even if you are not everyone hears it at all I never had, but the old streamers apparently do. If Roon is the transport mechanism to the streaner then it’s sending raw PCM direct to the audio processing rather than a wrapped file. So there is even less for it to do as its in the native format it accepts. If your going via spdif then this is moot as the DACs not receiving a wav, flac file it’s receiving an Spdif PCM stream but that process itself may have some sq differences. It really all depends on the kit being used and really if you can hear a difference.