I think the best advise would be to set all speakers to large on the Oppo and sett them to small on the 990 thus letting the 990 do bass management. Using the 7.1 inputs, about all the 990 does is apply bass management and volume settings.That way yuor speaker settings ar applied for all sources, and the Oppo is doing youd digital-to-analog conversions.
I'm sure that Gonk will chime in on this, he's the Oppo expert.