Amp - Quiet left speaker issue

Off-Topic
1

The left speaker is more quiet and the sound crashes. It can improve (but not always) after 5 -10 min. More often I have to slap the top of the amp (Marantz PM7200 I bought 18yrs ago) to get the sound back to normal. The issue persists after bringing this amp twice to a local repair music instrument shop.
1/ Any idea what the issue is with the amp? (I tested speakers, cables and the issue comes from the amp)
2/ Could turning-off the amp directly damage amp parts/components? (instead of putting on stand-by with the remote control before turning off the amp)

Thank you for your views.

2

Since we are dealing with electronic components.They most likely will break at some point.

It might be the Power amp board…? Not sure of course unless making a component level testing.
Found a Marantz PM7200 (Power Amp Board) & (Protection circuit board) Upgrade Kit being available so it could indicate that there might be something.

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