I ran both Stageline N and the inbuilt stage on my XS3. I couldn’t reliably choose between them, but did end up running the Stageline until it was replaced by a Graham Slee stage. That was a noticeable change!
I did read recently that the XS3 and SN3 stages were different, in terms of the component mounting. I know the boards themselves are mounted differently, but the article said the signal path components in the SN3 implementation were mounted differently too. That might make the onboard board a bit better in the SN3 implementation than the XS3.
The main difference between the phono stages
in the two amps is in the quality of the components
used, but even in the Nait XS 3 we’re using really
high quality Burr Brown OPA2604 op amps, while in
the Supernait 3 we’re also using through-hole resistors
for improved resistance to vibration.
From an article @james_n posted here: Supernait 3 technical specs - #12 by james_n