You cant understand how cables can make a difference so you declare they dont. But, you can go on and on, endlessly, about the need for burn in.
Myself, if there were ever something I would attribute to placebo .... it would be burn in. I remember when I disliked those totally weird sounding Ultrasone 2500's. I was told they need 300,400 or whatever hours of burn in and the presentaion will be transformed. Well a few weeks ago ... I heard a well burnt in pair (600 hour estimate) and the 2500's sounded just as awful as the first time I heard them .... and this was out of three different setups. So much for burn in.
I have also tried a brand new 701 vs a 701 with well over 500 hours burn in. I twisted the cords out of two completely identical setups and I could not tell them apart. The only hint they were different was the feel of the pads. But, if I ignored the feel of the pads and concentrated on the music I just could not tell them apart. Burn in, IMO, seems to be claim put forth to make people keep a product they dont really like in hopes their ears will finally come to accept the products aversive presentation.
Furthermore, back to your original statement, I have a couple different digital cables I am using right now with three Zhalou dacs and there is an unmistakeable difference. One, cable I like and one I dont. As soon as I put the non-preferred cable in the dynamics compress and the sound seems closed in and canned. I dont know how such a difference could be any easier to distinguish. Why? No clue .... but I have repeated the same preferance at least six times.
The moral of the story? Skip the preconceived notions and theories and to take the time to listen.
Now, even if the physical evidence were not so obvious to me, and were not so widely accepted as it is, I have tried personally speakers, several kinds, also the CD3K new and old, HD650 new and old, Editions new and old, 2500 new and old, Portapros, new and old, among the ones I recall. In all cases, of course in ones more than others, there have been differences, that after sometime they have vanished, and I'm not the only one that believes that, many others have experienced that effect as well. It is far more noticeable than the differences in the analog cables, which I have a really hard time hearing, whiles those I do easily...
Now to like or not a particular sound, presentation, etc...has very little to do with the burn in, the taming of the piercing highs you were complaining about is indeed, but not the liking or not of the sound at the end.
Maybe the 2500 were not your cup of tea, as the K701, HD650, nor any Grado are mine...Also as you say it is true that the "brain adjustment" to a given sound, has a role in the "burn in", we all are aware of that fact as well, but there is a true component, a physical easily discernible one, and that is not new for anybody neither...
Now you say that I can't understand how a cable can make a difference, but it seems that you guys are trying to make a very complex science out of a cable performance. Not sure if you are aware that cables have been around for maybe a century now, and there are hundreds of studies about them, and thousands of companies that produce them and live out of them for decades.
Now they seem to make a difference bigger everyday, and by coincidence, after hundreds of shady companies that promotes them with no facts, and live of those profits, had emerged, but the fact is, that till now none of them, and nobody in this whole green round planet has been able to show a single evidence of these differences, or the so claimed better performance. All tests we know off, have been miserable failures, and you still insist in hearing differences? And not even in the analog domain, that to me, could be more understandable, now you want to hear them in the 0-1 domain as well??? That is IMO simply absurd...
Sorry but I have better things to do that argue about this absurd topic, if you want to keep on going on this argument, please simply chose another of the hundreds of targets you can find here that will tell you exactly the same I do, as I will not go any further in the discussion of a digital cable difference...