Add External DAC or Upgrade to a New Streamer / DAC?

I2S is equivalent of SPDIF from clocking perspective and only appropriate when DAC supplies clock signal to streamer.

The only audio transport for Ethernet is AES67/Dante. There are not many DACs supporting it on the market and even less number of streamers. In Dante based networks clock distribution is even bigger issue and today likely requires ASRC feature inside DAC to minimize jitter.

@ap1 :

If there is not USB and you have to use AES/SPDIF/TOSLINK, then to deliver bits perfectly from streamer to DAC you need to choose DAC that has master clock OUTPUT and streamer with master clock INPUT. This is the only solution (other than USB) to make streamer slave to DAC clock.

Very interesting. Maybe I got this wrong, but you are saying:

1) If using USB from streamer to DAC, they are synced in terms of clocking, as the clock in the DAC is used

2) If using AES/EBU or S/PIDF or Toslink, then the clock on streamer is used, and no sync exists between the streamer and DAC

Is this correct?

For #2, what you are proposing as a solution is practically impossible (or hard at best). It would require the DAC to have a 10 MHz master reference clock output and the streamer to have a master clock input. Or both of them having a master clock input and using a third, external 10 MHZ Master Clock generator device 

I had the rs250 streamer/dac for almost a year and decided to add Gustard R26 external  dac.  The soundstage and clarity is obvious and  noticable . I can listen my music at lower volume with the R26 with better sound quality.  I sold the rs 250 for trying Roon with smallgreencomputer combo on the r26.  I don't accept the fact the hifi rose don't want to develop Tidal connect and they stick with their buggy software.  Even the free bubleUpnP is better than rose connect.  

I am not a roon fan  and Now I am waiting my eversolo dmp a6 streamer to join my R26 by using Tidal connect with I really like the AI for track radio and discover new artists. The nice thing about rs250 is its hdmi output, internal drive bay for music storage and vumeters.  You can have the same for half money with the eversolo streamer.

So yes, adding and external dac above 1.5k$ (r26, a26, pontus 2, etc...) to your rs250 will bring you to another level.

I'd like to thank everyone for their contributions to this discussion. I've learned a lot and have gotten some very useful information to work with moving forward - it's appreciated. 

If you want great sound at a reasonable cost get a MHDT Orchid DAc with a 6922 tube adapter and a used Logitech Touch.  Probably under $1700 and sounds wonderful using Pandora.  This combo was better than the other streaming services available in testing, not just someone told me.