Bryan, I always use DPLII or another of the ambience extraction modes when listening to 2-channel source material. As Dean pointed out, these modes have nothing to do with the relative level of the subwoofer or other bass material. They detect and extract out of phase material that represents reflected surround ambience, then send it to the surrounds.
I've never experienced anything unusual about the bass levels when using these modes. I wonder if it's actually the sub level or is it the surrounds which are causing the "boomy" effect? I never use the Panorama setting, since that simply takes some of the material that should stay in the front speakers and sends it to the surrounds, sort of a partial step toward "all channel stereo" which I don't care for.
I'm not familiar with any quirks in the 990 which might explain this, but again, it isn't the DPLIIx processing as such. Are you certain that when you were listening without surround in effect that you didn't use a "direct" or "pure" type of mode which didn't send signals to the sub when 2-channel material was being played?
