Ah, Armaegis. I see. He used his COP housing for his t50rp drivers, so he was left with extra COP drivers not in a housing. I think he probably already had extra DT bodies, so he put the COP drivers into those. It's more than likely that it was not because that they were a particularly good pairing, just that he could was all.
You should use the Beyerdynamic foam inserts. Thought, if you want you can test between no foam and foam, depending on if you want more or less treble respectively.
Lyova Margaryan of Tru-Fi Speakers
And my second question still stands if you know the answer (pardon my rookie question, i haven't modded headphones before):
Also question regarding the Alpha pads: Do i need to change only the cushions and not the included foams? I mean the Alpha Dog cup foams shouldn't even fit the COP/770, right? Can the foams even be separated from the cushions with the Alpha Pads?