Hi,
I had both machines, the Nad c542 was warm sounding, full bass, rolled off top end but more natural sounding than the CA 640c v2. However the 640c is leaner more punchy and more open in the mids and top end, it however can be a little forward and clinical sounding on some music (classical/jazz). If you after more relaxing sound go for nad if more excitement the 640c. The 640c was fatigueing on my system after a while and the nad lacked a little in detail. The nad is probably more balanced however probably not for detail freaks
I tried the Arcam cd73t that was a step up but boring on some music like rock. I ended up buying a consonance cd120 linear - musical fluid sound, that was detailed, open, smooth and natural sounding.
Cheers,
Steve