If youre going for the 685's then i'd say that there wouldnt be that much in it if you added a power amp IMO
BUT
If youre going for the 683's then get the power amp, plug it in, sit back and enjoy!
The 683's will lap up the NAD power and spit it back at you with vengeance!
My mates got the same system apart from he's got the NAD C352 with a NAD C272 power amp and the B&W 683 speakers and a Rega Apollo CDP and the sound is stunning.
The bass weight is scary; i really thought he had his PV1 plugged in it was that deep and powerful! BUT... The bass is fully under control and its a superb listen, more so if youve got some of Masters at Work cranking through it all