@kgt
I did play around a lot with the sub settings on my (now to be replaced) Marantz SR9600.
As often advised, I've set the crossover to 60Hz and my center and rears to small, although these went down to 45 Hz.
But I didn't like the sound (bit mushy and thin) so I stuck with 80 Hz, all bass to the speakers PLUS to the sub.
This sounds best, and I guess it'll be even better because I believe I can set the 990 to different level for the sub in stereo mode.