There’s no unity gain option on the NAIT3, so while you can connect through any line level input, you’ll need to set the volume at a useful level on the NAIT3 as it cannot be bypassed. Once the volume is set (try around 12 o’clock), you can of course then control overall volume from the Arcam, but we careful if switching back to using other sources on the NAIT, as the volume level maybe set quite high and need lowering.
Worthy of note… Default volume settings on your Arcam use the same IR codes as Naim stereo amps.
If your Nait 3 isn’t the R (remote control) then no issue…
But if it is an R, the Naim volume will change when the Arcam changes… and as they both go up and down together the effect on the nait will be exponential and a huge problem
So you’ll need to go into the avr550 manual to customise the remote (unlock the AMP mode), access the advanced settings menu, change the IR code to system 19 (16 is standard), and change the arcam remote to the alternate or codeset.
Any Naim dealer with experience of Arcam AV will probably be able to assist, or Arcam tech support if you get stuck.