A lot of this is apples to oranges comparisons, best ports on streamers and dacs make direct comparisons impossible. One should purchase dac and streamer as a package, decide in advance which scheme they're going with, AES/EBU, USB or I2S, both dac and streamer should be optimized for that scheme. I'll reiterate usb most difficult/expensive to optimize, has more inherent disadvantages, usb can have extremely high sound quality, gets bad name from less than optimal implementation in both streamers and dacs.
In question above as to how N20 would compare to Zenith and Phoenix. I'd keep Zenith + Phoenix vs N20, K50 and MU1 IF STAYING WITH USB. Now, if changing rendering schemes anything goes. USB is capable of wonderful rendering, Phoenix is one example of that. Now, is that Phoenix going into dac with optimized usb board, that being XMOS or Amanero, perhaps some proprietary board. If that is the case one isn't going to do much better using another rendering scheme. Assuming the Phoenix going into dac with great usb implementation, there still may be gains to be had with a better streamer serving only server duty. In other words Zenith II may be bettered with newer Zenith or some other streamer.
Point I'm trying to make above. Streaming sound quality can also be improved through better streamer acting ONLY as SERVER. Lets say one already has optimized usb rendering via Phoenix or another dedicated usb streamer only, assume optimized usb implementation in dac. One can still achieve better sound quality with higher quality streamer, and NOT USING IT'S RENDERING CAPABILITY. Improved noise suppression, music player processing ALONE can make substantial improvements in sound quality. I know this from my own experiments with multiple rendering schemes VIA USB.
I have no experience with I2S and coax rendering so above may or may not apply. At one point thought I'd experiment with I2S via DDC, in the end made no sense considering extra complexity, limitations of DDC equipment itself and less than optimal I2S implementation in dac.