To my ear (and not only mine), NDX2 on left and XPSDR on the right, and touching, was audibly not ideal (a bit vague and not ultra-quiet). Even 20cm gap between them was enough to improve things. Even ‘digital’ boxes seem pretty sensitive to these things.
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“Sometimes I think that if you cannot do it properly, you may as well just stick with the more basic system in the first place”
This 
My first system was CDX/102/HC/NAPSC/180 (similar to what the OP has) and I used to stack the 102 on top of the 180 and the HC on top of the 102. The CDX was to the side and the NAPSC was on the floor. When I did buy a rack several years later, for the life of me I couldn’t discern any difference in SQ.
According to the Naim manual (I have highlighted the area of interest):
3.1 Siting The Equipment
In order to reduce the risk of hum audible from theloudspeakers, power supplies and power amplifiers
should be located a reasonable distance away from
other equipment. The maximum separation distance for connected equipment is that allowed by the standard interconnect lead.
There is no mention of stacking degrading SQ. In addition, it does not even caution against stacking. It would appear that in some instances an audible hum may be heard when power supplies and power amps are situated close to other boxes, but only in terms of “reducing the risk”.
I’ve been to several dealerships where I have seen boxes stacked. The photo taken by Grx8 on a recent visit to Focal Naim in Lyon also shows what is perhaps an NDX2 on top of a XPS (as shown below) 
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That phono stage & PS might be the best-looking Naim boxes in the current line.
Interesting as our isoblue racks give nowhere near that gap. Maybe wood stops transformer flux? I don’t know the answer so just speculation
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These last few responses are helpful thank you. Before I start trying to reorganise, any comment on my guess that if one of the two components has to go on a normal shelf versus the acoustic slab, I am better off putting the NDX2 on the slab and the PS on the normal shelf?
What is an “Acoustic Slab”?
Although not recommended to stack, if you do so the key thing is ensuring you have enough ventilation between components. Power amps, streamers and power supplies can all get rather warm and need good air flow around. NACs don’t get warm, but ironically probably benefit the most from physical separation.
Also consider cabling… unlike regular commercial cabling practice and network cable practice, the cables on Naim devices are best connected to hang free under their own weight, this is for mechanical (sound) decoupling, and optimum connection.
As far as transformer flux, the cases are faraday screened, the layout is harmonised across full width products with inbuilt powersupplies and so vertical stacking is not particularly problematic (though not recommended) and there is some coupling through the cases… hence no doubt why the full width internal layouts are harmonised other than for NACs which don’t have inbuilt powersupplies.
BTW a lot mumbo on jumbo spoken about the need of physical separation of the NAPSC… I understand even Naim are bemused on that one… it’s one of the most benign powersupplies Naim produce… and rather straightforward … just bear in mind being not full width it doesn’t follow the harmonised layout … but its transformer is relatively very small so doesn’t really matter.
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great answer, thanks.
The best test is to blindfold the listeners, then experiment with stacking and separate shelf power brains and brawns. I am sure 90% wouldn’t notice the difference.
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If you really want to optimise things, it would be a case of brawn/brain right/left split, or keeping preamps vertically separated from power supplies by introducing empty shelves between them. It’s all about how far you want to take it. In my case, all three boxes have transformers, so the closeness really doesn’t matter much.
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Ah yes, the much maligned little box.
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If you want to avoid rack and also stacking, this is a good option. But the problem may be the cables not hidden.
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That’s just a static display in a retail setting though?
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Adding the cables to that will be a right mess
It’s the same with marketing material for speakers, they show them looking amazing in a contemporary lounge setting and without any cables connected to them. I’m guessing they aren’t using NAC A5 in these fancy Naim/Focal boutiques.
I rest my case.
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Maybe if the cables are hidden by a furniture under the shelves….
The picture is from Focal powered by Naim shop.
They do the same with bathrooms etc. The times a customer has said to me " the one in the showroom didn’t have that big pipe at the back"
You just have to laugh
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a manufactured stone with an internal honeycomb structure - very cheap (they are pavers), heavy and inert but still manage to look OK - grey on a white cabinet top.
Thanks for letting me know