Just a thought, what is the setting of the two front L&R speakers on the processor end if any? Is it "Large" or "Small" ( please consult your manual)? Have you tried configuring the settings from the processor? Is there a "direct" settings (2 mains L&R) only and have you tried those?
As for the amps, you can compare their sensitivity rating for maximum output. It is possible that the 990 have a higher sensitivity (lower input voltage required) and if so, is there a way to limit the output from the processor side?
As for the amps, you can compare their sensitivity rating for maximum output. It is possible that the 990 have a higher sensitivity (lower input voltage required) and if so, is there a way to limit the output from the processor side?