Contrary to your thoughts, some dry joints often cannot be easily provoked by tapping, but instead only appear as the temp of the component increases and its legs increase in length as a result.
I can't remember, but if the bottom panel removes to give access to the soldering under the main amp board (next to the big alu h-sink), you may be able to inspect it yourself. Obviously don't have the unit plugged in, and leave it a few mins unplugged before working on it to allow the big caps to discharge.
If there is a small 3 inch square board stood vertically then the faulty soldering will be on that. See
repair or replace rx500b
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