Some more photos of my NAXO 2-4 in original condition and various states of undress
here
I think the photos DaveMc64 links to are correct and show the cap orientation correctly as -ve to the ground plane.
The Prosimic are not that cheap, they are not stupid expensive either, if they are what Naim still use (I am unsure rather than saying one way or the other!) then they are a good choice from resale perspective.
Axial caps are harder and harder to come by. The originals are Roederstein, they were bought out by Vishay, Vishay give guidance on their replacements for old Roedersteins and still make some axial caps. These would be a good deal cheaper than buying the Prosmic from Intune and perfectly adequate replacements. There is nothing special about either the original or Prosimic in terms of ESR which is what might help in these PSU filtering positions. I know, I have tested them
Vishay say the replacement ranges for the original EBU bipolar are 137 ABA, 137 92 AB bipolar, these in turn are discontinued. Vishay state "Low market demand and no further material availability". Vishay suggested alternative for the non-bipolar general purpose EL/EB/EG Roedersteins are Vishay 030/031 AS, 041 ASH. So something like
this.
If you want to save a few £ or improve in there are other options.
The relay timer circuit anything 47uF and at least 25V should be fine.
Many people use Nichicon Muse UKW or UKZ in Naim recaps, these are radials but can be fitted across the axial holes OK, something like
this or
this.
For the rest I would consider a polymer aluminium electrolytic. The old acoustica favourite Panasonic OSCONs are obsolete, but there are other manufacturers. Again 47uF and at least 25V, these will be radial again. Polymer aluminium are orders of magnitude lowerer ESR and should give some improvement. Something like
this. YMMV.
As you will see from that thread I went the whole hog and removed the 27R/47uF PSU filters and put local ALWSRs in. In some ways this makes the NAXO better than a SNAXO (local regulation better than remote regulation would be the argument) without requiring a SuperCap. Since I can't compare my HiCap NAXO to a SuperCap SNAXO it is an unsubstantiated bold claim, but this is hifi so I feel duty bound to make such.