The transport in a streamer DAC is as important as the quality and accuracy of its analogue reconstruction.
Therefore I strongly recommend you don’t skimp on transports… Naim in my opinion make some of the best transports currently out there… like the NDX2 and ND555… yes it’s diminishing returns… but in the latest streamers the transport was the area of most innovation and advancement and a significant improvement over the earlier Naim streamers.
Therefore I say NDX2 as a transport (though some on here use a ND555 as a transport) and the DAC you prefer.
Naim use the SPDIF format for linking to DACs, yes whilst ultimately bandwidth relatively limited to 192/24/2 transport framing, has the potential of being lower noise than USB that some other DACs use.
In my experience (and through my applied engineering theory and practice) there can be significant merit in system decoupling the DAC from the transport… now although Naim go to a significant extent to internally shield in the ND555, there will be benefit in the limit to separate.
You will find with decoupled DAC / transport the effects of Ethernet tweaks like Ethernet/streaming leads, FLAC vs WAV, ethernet switches etc all start to have much less of an impact… especially with the new Naim revised architecture streamer transports… which to my mind is how it should be… and leads to a more deterministic performance.
Now with the new streamers some argue that not using its inbuilt default DAC is a waste… well i suspect it’s not economic for a Naim to do two variants, so they combine a model that can work in either mode… specifically the streamer is designed to be internally configured as digital transport mode or analogue mode but not both… and in some ways disabling the inbuilt DAC is no more a ‘waste’ than disabling the internal PSU when an external PSU is used for the NDX2 for example… these are all implementation choices.