Investors in AA-rated classes of subprime MBS deals may soon face new competition from Freddie Mac, which is eyeing a move down the rating tree as it searches for more yield. “We support the subprime market though our portfolio. We are currently looking at expanding [our portfolio purchase] beyond the AAA market,” said Don Bisenius, Freddie Mac’s senior vice president for credit policy and portfolio management, during a panel presentation at the recent Mortgage