Here’s the connection diagram for you. Note the direction markers on the SNAIC and DIN-XLR. Just replace NAP250 with NAP300 and connect the second DIN-XLR in the next door DIN4 on the HICAP.
You should check the current cost of a service with your Naim dealer. If you’re in the UK then there’s also the option of using Class A, who are authorised by Naim to do servicing on Naim equipment and use Naim parts and processes.
As for DR upgrades, these had to be suspended and are not currently being offered.