Hi!
A friend of mine bought the NAD C541i receantly and it really sounds damn good, especially regarding the price. He has compared NAD and ROTEL amps side by side and the ROTEL clearly has Won the "Contest" (are also a bit pricier). So it may be a minor sonic compatibility issue with NAD amps and B&W speakers. By the way, he has B&W DM602s2 speakers. And please give these speakers room to breathe - at least half to one meter space between them and the back wall. Perhaps a bit of cable "rolling" can help before selling and buying new...
Cheers!