I have now tested both units in the same car, just swapped them. Same
everything else. How do you know it is not true? I notice the difference,
my
be the newer one in not performing 100%

I know it is not true between working units because there is no difference
in their electrical signals. This can easily be tested with distortion
analyzers and spectrum analyzers.

So if there's no difference in electrical signal, where would you suspect
the difference comes from?