Mrtennis, your comment is valid although I would suggest it is only a step toward what could be a far more pleasing experience.
If you liked the sound of replacing one cable/pair did you then proceed to add additional cables incrementally to see what the effect was? i.e. transition toward a new complete set?
I think can understand your perspective in the matter based on what you consider to be good sound. We will differ on this, but I believe your view is that good sound is not overly detailed, but is warm to syrupy sounding with a higher emphasis on tonality than definition.
If that is correct, and not judging your opinion in the matter it is entirely possible that by mixing the cables you are able to dial down the definition and thereby have the tonality come to the fore.
My philosophy in regards to defintion/detail is at the other end of the spectrum; I say that one cannot have too much detail/definition in a system and that if the perception is such that means there is a problem with the system, often times with the tonality. So, one of my goals is to seek cabling which is extreme in defintion/detail, however I will not sacrifice tonality nor any other parameter in order to achieve it.
So, yes, if you are wanting to avoid an extreme amount of defintion/detail then mixing will very likely allow you to do so. However, I believe that if you were to follow up by testing other cables singly and noting the difference in sound you might be able to move toward a brand which would be far superior in every aspect of performance you desire.
I continuously attempt to mix in one or two cables to see what the result will be on review rigs. Without exception because I am using a unified set which is extremely agreeable to my ears no mixed sets have been as appealing. In every case there has been a diminishment in detail/definition. I believe this means I have discovered a brand of cables which is to my ears "right up my alley." I also believe any audiophile can find a one brand set which can be so in tune with their sense of aesthetics that they would prefer it to any number of mixed sets of cables.
I am not stating that in every instance every audiophile will find a single set of cables superior; I am saying that mixed cables do not afford a clear route to guiding the system toward the desired end result, and that if one persists with testing complete sets they will likely discover an entire set which is far more to their liking than any mixed set they have previously used.
Can I "prove it"? No, no more than one can prove vinyl is better than digital. But I believe there are methods which can move a rig toward a goal with more certainty.