Stay away from the nad

djengan,

I suspect that the problem your having is simpler than they say.
Two questions:
Do you live near salt water?
Do you own a tape deck?

The description of the problem does not suggest that it is serious.
I'm guessing you don't use the tape monitor switch. It will eventually cause the audio to "drop out" or drop in volume if it is not used especially if you live near salt water. 

The cure:
Exercise the tape monitor switch a few times and see what happens.
If there is a "tape 2" switch - same thing.

Hope this helps
Roger

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