I presume that it's a C160 you have?
I actually sold this unit through ebay and the buyer's insisting that it's not 100% o.k. and he would like his money back or reimbursement for repair of the unit. I suggested he use another input rather than the tuner or turn the tuner off when not in use, but he refuses. NAD's been no help, telling me that the warranty's up and the unit is now out of Canada and in the U.S. so any warranty is not transferrable. They also refuse to acknowledge that there's any chronic problem with the unit, even though it is mentioned on some consumer reviews that they did realize intially in production that there were problems with capacitors, which caused the switching problems, and that they did change capacitors to rectify the problem.
I never had a tuner plugged into the preamp before I sold it, so guess I'm kind of screwed on all this.