Channel Imbalance at low volumes

It can be dried out off brand capacitors, dirty relays, dirty pots, dirty switches, and so on.

the unit is from 1986, and was made in Taiwan with off brand capacitors.

It’s old and it’s tired.

It needs a cleaning, check out, new capacitors on many spots, a general check out by a competent technician.

Nobody gets to luck out on a unit that old. They all need service, even one in a hermetically sealed perfected environment since 1986. (stereo cabinet, out of the sun, no heat and a level room temp and humidity environment). Even that ’perfect’ unit would be in trouble, in some way, just from the heat of being operated the odd time and general environmental aging.

I've  had my hands inside probably 3-4-5 of that exact model, never mind the other many many dozens of times I've handled a NAD product at that level (service).

Impossible to say. Unless it is a full proper rebuild, which might cost $200+, then it's all going to be crutches of a sort (good crutches, bad crutches)....and the lifespan of the amp will be what the clock says when it finally fails.

This is the nature of audio electronics.  Like life, it's a series of blind and open trade-offs.