Something else must be in play here. A 1000 ohm output impedance in itself is meaningless without knowing what the input impedance of the associated gear and the capacitance of the cable is. Neither of these are an issue with typical gear you would connect the 990 to, and certainly would not be an issue with an Outlaw power amp.

Have you performed any objective measurements such as frequency response? If the response were effected by a component mismatch, that would explain the change in sound quality.