Look to what is driving the output tube - both the power supply feeding it, and the upstream components in the output section (voltage regulator, etc.).
Cost will be dependent on that. I would think $250 on the low end, and potentially 3 - 4 times that on the other end of the spectrum. I know that's a wide range, but I don't know how Audio Note components are constructed. Whether discrete components or an entire board(s) needs to be replaced determines a lot, as does the time/labor involved.
As Davidbigeye said, the unit should work fine through the digital out. And, as Rbrowne said, contacting the service rep is the right move.
Cost will be dependent on that. I would think $250 on the low end, and potentially 3 - 4 times that on the other end of the spectrum. I know that's a wide range, but I don't know how Audio Note components are constructed. Whether discrete components or an entire board(s) needs to be replaced determines a lot, as does the time/labor involved.
As Davidbigeye said, the unit should work fine through the digital out. And, as Rbrowne said, contacting the service rep is the right move.