Sounds like the Nak is dying on you. Time to fix it or get a new receiver...
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It sounds like you have a filter cap in the power supply that is dying.
When the electrolytic caps started to die, they are leaking current. The leak causes internal heating in the cap, which causes more damage to the cap, so once the problem starts it progressively becomes worse. It's called an avalanche failure mode because it starts small, but it builds on its own, like a small pebble can start an avalanche. Then the hum will get so bad that is becomes unusable or or the digital logic in the receiver will start to continuously reboot.
The sound is dimming because the leak in the power supply is causing the supply voltage to slowly fall, then the amps go out of bias, then the amps draw less current, then the supply builds back up quickly, and the amps "pop" as they come back into bias again. Then the cycle repeats.
As I said, it will only get worse.
The up side is that leaky caps are easy and cheap to replace. You may not be able to do it yourself, but the guy at the repair center will probably have is done in 20 minutes.
If you take the cover off, you might spot the guilty cap because it is likely to be a can type of electrolytic, and it is likely to start to have become discolored relative to it's neighbors, or it may look a bit puffed out, like it is trying to explode.