Stay away from the nad

People love NAD your breaking there heart oh boy!!

It has a built in timer :-)

Over the years I've owned several NAD components, back to the 1980's and have had minimal problems with the brand.  A receiver I bought years ago had that same problem (one channel suddenly about 25% of the volume of the other), and my M5 Masters Series SACD player had a transport failure (wouldn't open the tray), but other than that, good service from NAD components.
Having said that, I understand that NAD was one of the first audio companies to outsource their manufacturing to "contract" builders.  It would not surprise me that, if you opened two of the same model that the components would be from different suppliers.
I have since upgraded to electronics designed and built in the USA (McIntosh & PS Audio)...and so far (knock on wood), haven't had service issues with either brand.

djengan,

That particular model has 3 switches in a row that carry the audio signal on to the power amp section. 
Tape 2
Tape Monitor
Tone Defeat

These switches are rarely if every used so there are 3 chances that a corruption can occur. It will manifest itself with the exact behavior you describe. Anywhere from apparent shift in balance to missing a channel entirely. I have had several different brands come in for repair (years ago) that can be "fixed" by just exercising the switches.

Anyone else who currently uses something similar I would ask to try this - without touching the switches put on some high quality music and listen for a minute then  "exercise" the switches that you don't use and go back to the beginning of the track and listen again. It will sound much better and have greater detail and it didn't cost you a "ripoff" repair bill..

When the purity of any connection in your system is not right this happens. Interconnects that have sat in one position for months have the same problem. If you simply listen to music then twist the RCA connections by a barely detectable amount (enough to break and re-establish a better connection) your system will sound better guaranteed.

Roger