Don't raise the output of the MA6500 by lowering the impedance. Reduce the output. Zu Druid Credenza, 12 ohms. I've heard the combination and it makes the MA6500 sound much closer to the musicality of the iron output MA6900/7000, while still having plenty of power for the 101db/w/m Credenzas (high efficiency with lots of power handling).
Phil
Phil