chebby:
I am suprised that Naim is new to you. (Especially given that as a long time Arcam user you are obviously serious about hifi.)
They have been around since 1971 (based in Salisbury, England) and have been prominent in the world of 'specialist' hifi for most of that time.
I will not make any Naim vs Plinius judgements because I have not heard any Plinius kit.
If you have always used Arcam equipment then Naim is going to be one heck of a shock to you.
To some it is refreshingly musical and rhythmic with great timing and honesty. It is not 'sweet' or smooth - unless the recording is so - and it doesn't do all that... "Treble as light as the fluttering of an angel's wings" sort of nonsense.
If you enjoy hifi 'tricks' like being able to identify the precise direction (and makes and number of occupants) of distant cars faintly heard during the quiet bits of a classical recital then Naim may not be a good idea.
If you prefer to be inspired to get up and play air guitar/drums/violin/trumpet or whatever and thrash around like a loon dancing (or 'conducting') during your music then Naim is a good idea! It suits all genres of music and manages to grab all the most important musical cues by the scruff of the neck and control them effortlessly.
If you want audiophile 'twinkle dust' and spookily holographic imagery or 'warm, chocolatey and smooth' then I am not sure Naim would be right either.
Paradoxically, I did not like the Naim sound for a long time (even the stuff I now own for the first couple of times I heard it demo'ed) because I had been conditioned to expect that things like 'airiness' and pin-point 3D imagery and dark 'cavernous' bass were the trademarks of 'real' hifi.
After getting this dreaded Naim stuff home to demo, I very quickly realised that "smooth", "airy", "chocolatey" and "dark" were only really good qualities for an Aero bar and not hifi. When it just comes down to the important elements of music, Naim is superb.
Horses for courses Chebby, personally I want imagery in a decent hifi system - but didn't you claim your Naim kit has considerable depth as well?