aniwolfe,
Maybe you can tell me what is happening inside during the 300 hours?
One possibility is that there are physical changes occurring within the cable over that time that alter the sound in a way you are able to hear.
However, that boutique digital cables "sound different" than basic, capable digital cables is highly disputed by engineers and many people who know about computing. Let alone that a digital cable "changes sound" over time. So from what I've read on the cable debate, the plausibility technically speaking is wanting in such claims.
Another possibility is that you imagined the difference you "heard."
This has plenty of plausibility - endless studies on the malleability and unreliability of our perception support this possibility.
But...many audiophiles don't like confronting those facts. Makes things inconvenient for some portion of the hobby.
Again: I am not rendering some absolute judgement on why you heard what you think you heard - whether it had an objective or purely subjective basis. I'm just pointing out the case for skepticism. You are not at all obligated of course to change anything in your purchasing behavior based on this. If you are firmly wedded to a purely subjective method of trying to understand audio, this will fall on deaf ears ;-)