I've bought several different houses over the years and an inspection, including electrical, is always part of the buying process. At least two of them had a few sockets wired incorrectly. I've simply offered the idea as a something to check as it is easy to do and the devices are inexpensive. Of note is that every Wiim amp the OP has seems to have the problem to some degree. If a design flaw there would certainly be a lot more complaints. And it seems unlikely the OP is the only person who received multiple defective units when there have apparently been no other complaints, at least that we've heard of. Given those factors, the problem seems likely to go back to something about the OP's environment.There are just two wires that can be switched back and forth at the amp every time. What kind of error exactly could a wall socket tester detect that would lead to what the OP is describing?
I've offered my two cents, so the OP can use it or ignore it as he sees fit. No need to beat the issue into the ground. Hope he finds a solution.