I know your cats are indoor only, but have you ever considered Revolution to ward off the earmites?

Our boys are in/out and each of them gets a dose once a month year round. Two of them had earmites shortly after we got them, but since using the Revolution not a mite in sight. I do not use it on the indoor only girls, but neither of them ever had mites.

If you're interested, I can give you the info where I buy mine. It's up in Canada - no prescription needed up there, and I save about $300/year buying from them.