I note that the 990 does not let me specify no center channel. However, for Dolby Pro Logic II and IIx, it allows mixing 0 to 100% of the center channel to the left and right channels. I consider this a bug: it should be possible to specify no center channel and have that signal mixed with left and right.

Not sure about the other modes.

Of course, the Tape/Out, Aux/Out, and Video1 Rec/Out always receive a two channel downmix.