Electronic products, for example: KZ brand headphones, after manufacturing, do they undergo any individual tests to prove that they are working correctly and without physical damage? Or do they only test some units from a batch?
I have some units sealed with plastic stored away
You would normally have to contact the manufacturer for that answer or if you get lucky somebody here may know.
Both! All depends on what the cost is. Is it more costly to not test and accept a few bad returns or is it more cost to test each device or is it more cost to test each device and have few returns? You make the choice.
I expect that only professional grade cans are tested individually.
And they then would then come with individual frequency test graphs.
Highly unlikely that individual testing is done with consumer-grade products.
Leo..