But its built to be an streamer, so seems silly to buy it only for the dac element.
I would be using it as a streamer too, but only for Chromecast Audio, none of the other streaming formats/services. On that subject, from what I can see online, one downside of the 851N when compared to the CXNv2 is that the 851n doesn't seem to include Chromecast Audio support.
In any case, I think this may be a very good point:
The CXN is a full-size component which is a network streamer with a large colour display…..and a built in DAC. Think how the money has been spent on the various parts. Then compare it to a stand alone DAC in a smaller case where the money has just gone on the DAC and quality internal components.
If a large proportion of the ££ has gone on features I'll not need (rather than about £50 out of the £800, which I was probably naively assuming it was!) and so I should not get distracted from my original plan A of an M-DAC+, except to perhaps consider Tinman1952's suggestion of an SMSL one.