I do not know whether Ultra or CXA81 has a better DAC but I can tell you the following:
- A streamer will pick up the highest quality your subscription allows (in this case lossless - CD quality - on Spotify). You will then be able to decide what to do with that signal. One option is let the streamer's DAC do the job. Modern streamers do a very decent job. I do not know whether the DAC in Ultra is better than the one in CXA81 but, if you get either Ultra or MXN you can decide for yourself. If you are using the DAC in Ultra, connect it with RCA. If you are using the DAC in CXA, use S/PDIF digital cable.
- I have a CXA61 (chosen for it softer performance over CXA81), a Wiim Pro Plus and Chord Mojo 2. The DAC in Mojo 2 is markedly superior to the one in either CXA or Wiim. Wiim Pro DAC is as good as the one in CXA61. Mojo 2 will outperform even the DAC in CXA100.
- Bluetooth will always give inferior sound to WiFi connection via streamer.
- MXN10 is an amazing streamer (look at Darko's review on Youtube) but Wiim Ultra is on a par. Either of these would do an amazing job.
- Make sure you set the respective apps properly in terms of connections, max audio quality, etc.