Beware of the cable claiming long burn in period.

Not allways true , like a good capacitor ,cables especially with 
Teflon dielectric can takewell  over 300hours for sure . Just ask          VH Audio for example. Theirs caps ,as  well as Teflon Litz - OCC Copper Wire ,and caps.
we have documented times when I owned a Audio store and within the last year. it is one of the exceptions to the rule.

I realize it’s frustrating but even after the arduous and supposedly magical 300 hour mark is reached there is still a long way to go. Proof? Hook those cables with 300 hours on them up to a real burn in device like the Audiokarma Cable Cooker for a couple days. Then hold on to your tu tu. 

+1, Geoff. Just get a cable cooker. It speeds up considerably the entire burn-in process. It can be used on all types of cables and even can be used to burn-in capacitors.

But even for certain cables (such as the Teo Audio cables) in which the manufacturers don’t condone use of active burn-in cable gear, conventional in-use audio burn-in is usually sufficient. 

@prof-    1) That cable burn-in occurs, has been established by the manufacturers, as well as those multitudes, that have provided their empirical evidence. The, "dispute" is in your mind(et al). 2) The Scientific Process allows for/depends on empirical evidence, whether that matters to you, or not: https://www.livescience.com/21456-empirical-evidence-a-definition.html 3)  I asked, "Perhaps you can tell me, WHY that can’t be a cause(or, "plausible"), SCIENTIFICALLY?"    You could have just said, you have no valid, SCIENTIFIC reason to doubt the plausibility of my conjecture regarding Dielectric Absorption, only your biases. But, who would expect that?

Try using cables from reputable manufacturers who have a history in the industry. If you don't like the cable, the sell it on Audiogon, eBay or US Audio Mart. Try buying your equipment and cables/accessories from a reputable local dealer with a showroom. They can switch in and out for you so you can tell the difference.  Nothing to do with which equipment it is. This isn't speakers and room interaction, it's getting information from source to pre-amp. And if you have the ability to Exchange (no one should give you anything but store credit as they then have an open box product they will lose money on) then respect the terms.  You don't need to listen to anything while breaking in.  Turn your volume to zero or mute and run 24x7 for at least a week. You can, of course, listen as time permits.  Keep sending signal through the cable.  If you hit the end of the exchange policy and don't like it before it expires, call your dealer and tell them you'll be in to make the exchange. And just because a manufacturer espouses their virtues doesn't necessarily mean YOU will agree. But again, buying without auditioning is the risk you take. And if the manufacturer is selling direct means they have little credibility. They aren't using a manufacturing facility with QA process and controlling tolerances. And this is why there is a robust secondary market. The blind chasing a shiny object. So be it.