The change is more complex than the tuning pads, which act more predictably like tone control.
With the foam, ETHER becomes simultaneously less bright but more revealing, while ETHER C doesn't quite get brighter, but it adds more sense of air (OK, call it brighter, but it doesn't make the phone "hot" if you follow, as the gain in the highs is above 9K). Micro details really pop everywhere, even the bass, and yet the highs become better focused and smooth. It's subtle yet not.
Now that Captain Enigma has spoken, you will doubtless go to sleep thoroughly confused. But hey, at $9.99 + shipping we're sending these out at way less than it cost to develop and produce because I wanted there to be no reason for owners of a headphone in this class to not just try it. It's really worth trying, and while I can't promise everyone will like it, most will.
Worst case is you disagree and burn $14-25 depending on where you live and you decide to take it out... So far nobody who's tested it has expressed any desire to go back.