The A18 is a good amp. A nice rounded sound, slightly on the soft side at times but never uncomfortable to listen to.
However, if you like the rhythmic drive and involvement your NAD gives you, don't buy the A18 as in that respect it will be a retrograde step. I've got an A18 but I've actually ended up with a Rotel RA-04SE as well and, the majority of the time, I prefer listening to the Rotel for its greater level of involvement, detail and energy. The Rotel gives me everything I used to like about my NAD but with a bit more control, particularly at the bass end of things. It also amazes me that the midrange is better on the Rotel than on the Arcam. In short, when I listen to the Rotel I focus completely on the music whereas with the Arcam it SOMETIMES feels like something is missing.
Personally I think your NAD is a great amp and to make a worthwhile improvement I'd suggest thinking more in terms of a Rotel RA-1520 or a Creek Evo 2.