I'd definately either have the vet recheck those ears or seek a second opinion.

If you're finding coffee ground type debris and the ears are very itchy, that's usually earmites. The vet may have missed seeing them under the scope last time, which is highly probable. At our hospital if we have those signs (even if we can't find the mites under the scope) we usually give the cat a dose of Revolution which takes care of earmites without doing drops in the ears (which most cats hate). The problem usually goes away then. Although in your case with so much itching there could be a secondary infection that would need treating, however if the earmites aren't cleared up then treating the infection will be a never ending battle.

Good luck.