we are an Nad dealer it really depends who the "authorized" repair center is many small independent service centers may be authorized but are not really equipped to service Nad at the same level the big repair facilities I would investigate who this facility is
the other option would be to use a Node with a good amplifier the Node would provide streaming, DAC, DIRAC, Bluetooth, WiFi, Roon ready, HDMI Arc, and the amplifier would provide the amplifier an perhaps have an even better dac
we have a few ideas on the amplifiers for you to consider.
Dave and Troy
audio intellect NJ
Nad dealer