I actually got rid of my CXA81 in favour of an Arcam SA20 but still have the CXN v2 and CXC.
I think the digital coax out probably offers the purest signal path.
Having said that, I have read that the CXN upsamples literally everything for its analogue outs and downsamples the data back to it's original sample rate before outputting it via digital coax and optical.
I don't know if that's true. It seems overly complicated and unnecessary.
As you mentioned, some people claim the additional processing creates issues with audio quality.
What I can tell you is that when I play music from my CXN to my SA20 via digital coax, the SA20 displays the original sample rate.
My personal opinion is that digital coax sounds slightly better but it's hard to say. RCA sounds very good.
What do you think sounds best so far?