Burn-in revisited

High voltage flash testing is an industrial safety test for the insulation around wires when they are made, to make sure that electrical faults don't allow the insulation to ionise and become conductive of high currents and cause harm/death, I guess. I would have to read more, this is just conjecture. I have no evidence.

Also, mechanically bending a metal isn't going to alter dipole arrangement.

Bottom line is I'm not a researcher so I have no idea about dipole moments in copper.

So if we consider that metals are fairly uniform in structure but when it comes to dipole alignment, there can be some variation (imagine a permanent version of Van der Waals forces in chemistry). If there may be subtle differences in the behaviour of charge and their motions and how they transfer signals down the length of cable so there could be a small difference in bias, caused as previously suggested, during high voltage flash testing when manufactured. Magnetic fields and electric fields are not simple things to understand but this is where I believe the explanation lies. I have personally been able to perceive differences in bass focus and phasing when I tried swapping cable direction in the past and I really don't think it was placebo.