Questions about residual valuation are likely to make servicing even more important for subprime market players in the months ahead. "Servicing, servicing, servicing is the key to residual performance," said Jim Callahan, executive director of PentAlpha Capital. Long an issue for subprime market players, concern over residual valuation was underscored in an ongoing court case in which Chase Manhattan Mortgage has accused Advanta of misrepresenting loss performance on residuals the bank