If you look at the internals of say an NDX2 you can see that the PSU section is quite basic - various transformer taps go through rectification and smoothing. No fancy DR regulator boards.
Many devices do include additional further regulation across various boards, or for powered DIN etc,
This is the naim way - you add an external PSU to increase the quality of the power rails and hence SQ.
The XPSDR has fewer DR regulators than the 555PSDR which was one reason for the cost difference (let’s not start discussing VFM again).
DR regulators can be found in the NAP250DR, NAP300DR, NAP500DR, XPSDR, PS555DR, 200DR (DR power for attached preamp, not the power amp itself), Supernaits 2 and 3 (DR for preamp only), Hicap DR, Supercap DR.
I was only looking around the forum to try and find a post from 110db about the regulation in the bare NAC332, as I wasn’t sure if that was DR vs some other regulator scheme, can’t find the post I thought I recalled so far!
With all due respect to Dr , it’s no longer new and probably just seen as a std part now in new Naim , no need to advertise ,it is after all over 10 year old development
The NSS222, 332 and 333 have a “linear PSU regulation” board, under other boards so not immediately visible on photos.
As naim explain on linked posts this regulation is not DR, it’s previously used (but reliable/tuned/understood) tech.
Whether the 222 has a single DR regulator for pre amp only, like the SN3 does, I don’t know.
But to get DR regulation on all power rails on NC devices you need the NPX300.
From a marketing perspective this makes sense. If the 333 (for example) had 8 internal DR regulator boards there would be no point/uplift in adding a 300 NPX!
I wouldn’t say it was quite as simple as that. It’s a bit like saying you don’t need a Hicap if you’ve got a 200DR because the preamp power supply in the 200 then provides DR power. Putting the power supply in its own box gives it physical isolation from sensitive electronics, and allows it to be substantially beefed up given all that extra space available.