Thinking about it now, if David really did become an Epic, his only logical power would be Power Absorption. He was after all, able to use Prof's power even after he was turned. One more possible proof is that the last time Prof gifted him, he was also given healing powers, which was never done before if I remember correctly. Another one is that the forcefield was different from usual, it actually protected him for most of the fight with Obliteration when the normal forcefield would have been done after just a few hits (remember the chase in Steelheart?). Maybe the Absorption Power explains why he had a lot more of Profs power at that time. Because they weren't actually gifted, they were forcefully taken. As to why he didn't have the corruption even though he was using his powers. Remember after Calamity gave him his powers, he looked into the water, remembered his fear, and just set it aside. Meaning he faced his fear and overcame it at that instant. That's why he wasn't feeling any corruption. Anyone agree?