The P183s have to come with their PSU, the CP850 or CP1000, period.
The reviews on this site seem to suggest that the X650 would be quieter than the CP850. Am I reading this wrong?http://www.silentpcreview.com/article986-page5.html
I mean you have to interpret also those data.
With a baseline accurate within 1dB, it's pratically the same having 10 or 11 or 12dB.
Muffled in the bottom chamber a 12dB PSU is also virtually indistinguishable from a 10dB one: I swapped an M12D with a X-750 inside a Raven, and after a year or so I can say it was a waste of money.
About airflow, the P183 has renounced to the stock bottom chamber fan (with reference with the former P180/P182) right for the availability of the CPX form factor PSU: we can argue that its external inline fan has a role in bottom chamber thermals (IMO often inline arrangement are to be preferred in Antec enclosures).
Moreover the CP-850 is the CPX clone of the Delta-made Antec Signature, one of the best server grade unit available, and if you need more than 250W DC it should be equal or quieter than the X-series (as Seasonic controllers has always been very aggressive).
The only serious drawback (a true Achille's heel) I see nowadays for the CP-850 is its substantially inferior efficiency: if it were not an issue, then I think the CP-series should be definitely preferred for this specific application.
Eventually, does the X-650 work good in the P183? For sure, but to me it's a waste of money to have somehow slightly inferior electrical performances and chamber thermals, with no substantial better noise level.