If you happen to push the spades of the NAC A5 connector a little bit too hard into the S-400's connector - typically, to overcome normal friction - it can happen that the two shells fall apart. At this point, your S-400 has become unusable and you will have to send it back to Naim. Alternatively, you can unscrew the base of the plinth and replace the connector if you manage to get a new one. Replacing the connector is a bit clumsy because the screws are really small and you need a very long and small Torx T8 screwdriver to tighten them. In contrast to the connectors that terminate the NAC A5 which are very solid, the connector in the S-400's plinth is a defective design, in my view.
I understood that Ovator speakers come with Ovator speaker pins (for want of a better phrase) which are to be soldered to speaker cables, and which have been designed specifically to fit the sockets on the back of the Ovator speakers.