I would not dismiss out of hand the experiences of those who say cable break-in can have an audible effect.
Cables have capacitance, inductance and resistance. All are measurable, and electrically form what is referred to as an LCR network.
The most common form of LCR network is a passive loudspeaker crossover.
Taking capacitance alone, it is known to vary based on age. If you research the current literature from capacitor manufacturers, you will learn that they leave the factory at a different value than they are expected to perform at in-circuit by the end user. The effect is partly age alone, and partly the "forming" of a capacitor once installed in a circuit excited by AC signals (audio waveforms).
For the age aspect, the caps leave the factory at a value that is expected to equal the marked specification after shipping and distribution and final delivery to commercial end users; most capacitor manufacturers assume this will take three months. So, the capacitor should meet spec after three months.
That is partly why most capacitors are rated at x value +/- 20%. Although the manufacturer expects them to meet the rated value at a much tighter spec at the three month period, if measured earlier or if measured after a prolonged period in the distributor's inventory, they will meet the +/- 20% spec but may not be equal to the value measured at three months.
Once formed, the value generally remains consistent over time at the operating temperature typical of the component they are used in.
Both effects could possibly be encountered by a cable "break-in" procedure.
The effect is assumed to be subtle and may not be evident in all systems, depending on the resolution capability of the system. The type of dielectric used and the geometry of the cable in question, as both greatly affect capacitance, may result in more or less change in the values and perhaps the sonics.
There are two other factors that may come into play with regard to cable break-in. They both are brain-related rather than empirically determined effects.
One is our memory of sound, which can be quite powerful (one day you may hear "something" that induces to you investigate if something is wrong with your gear, for example) but is not equally discerned by different individuals. Put another way, some people won't notice changes in sonics over time while others will.
The first group is unlikely to discern any "break-in" phenomena, whether it exists or does not exist. The second group is more likely to discern such phenomena if it indeed does exist.
The second factor is another brain-related condition. Whether we admit it or not, our minds work inside what is referred to as a "Belief System". It is formed within our subconscious as a sort of "short cut" that reduces the processing needed to carry on day-to-day. As anyone who has struggled to study a difficult subject in school, or to solve a puzzle, or take an exam, thinking is "hard" and takes energy. The brain is always looking for ways to reduce the energy it consumes by difficult thought.
Thus over time we form Belief Systems that control our behaviour day-to-day. The Belief System allows us to make decisions without reverting to difficult, energy consuming thought, by simply applying a sort of rule to the problem, the rule based on previous experience.
Thus if your Belief System is based on experience whereby you have not noticed much (or any) difference in cables, and thus holds that cables "do not matter" then you are unlikely to hear a difference during or after a cable break-in period, regardless of the actual effect.
Similarly, if your Belief System involves an experience in the past whereby you did hear a difference while auditioning a change involving only a cable, you are more likely to accept the idea that cable break-in might affect performance.
Finally there is consumer Psychology whereby we want to believe we have made good decisions and therefore we may want to believe we hear an improvement with a new purchase or DIY assembly. That does not dismiss the possibility there is an actual change, but it biases us towards expecting one.
Cable of differing construction do not measure identically; that is well established. What is not well established is whether the differences are audible, or at least audible in a high resolution system.
Anecdotal evidence seems to suggest they are, but there are those that say they have not experienced any changes. It is paramount that any audition be under good practice, such as cleaning connectors with DeOxit during the audition, on both the existing cable and the new candidate, and the components connected, as any new cable is likely to be in good condition and any existing cable is likely to suffer some layers of environmental or corrosive deterioration. Such deterioration is most certainly measurable and is likely to be audible, depending of course on the level of deterioration.
In the end it is up to the end user to determine what, exactly, they prefer to use in their own systems, and what, exactly, they feel represents good value in interconnects or loudspeaker cables. Do not rely on others to determine your preference; find out for yourself and trust your ears, whether they tell you there is not, or is, any effect. Your sound system is not built to please some far-off reviewer or manufacturer, it should always be built to please you alone.
A good reseller should allow you to audition any interconnect with a no or low-charge return policy. Some online resellers also offer an audition period. Friends may have cable you could swap out to see if you hear any differences or ideally, any improvement.
We must always keep in mind that the entire purpose of a system is to produce sound, typically music, in a way that pleases us. If it doesn't please us, it is of no earthly use.
