Freddie Mac appears set to move down the rating scale in its subprime MBS investments, moving out of its tradition haunts in the AAA space to AA-rated deals. “We support the subprime market through 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 last week’s Mortgage Banker’s Association National…