There are two scenarios on the 950 that are capable of producing "double bass": stereo bypass when the mains are set to "small" and the 5.1 analog input when the analog bass management switch is set to "off."

The Model 990 does not have analog bass management on its 7.1 direct input. Instead, it either uses the digital bass management or (if all speakers are set to large) bypasses any bass management at all. Therefore, there is no potential for double bass on the 7.1 direct input.

I'm not entirely sure how the stereo analog bypass behaves - I thought I knew, but some recent discussion is making me reconsider that. There may still be a potential situation where the mains get a full-range signal but the sub also gets a copy of that signal that is taken from below the crossover point for those speakers - this would create a double bass condition, but it would (as with the 950) be in a frequency range where the mains are presumably dropping off severely, making it more like 1.5x bass or 1.2x bass.