I guess this is a good time to ask... Gonk, I have tried both of the setups you mention here and, by far, splitting the rear channels (turning 5 into 7) sounds better to me.
At least on the SACDs that I listen to I have noticed less bass than on conventional CDs - I have Dire Straits Brothers in Arms in both formats and the Stereo-CD played at Stereo-7 on the 990 sounds almost as good as the SACD (I've had that CD for 20 years!). After spliting the rear channels for SACD 7.1 direct, the bass appears to have increased. Do you think this is an illusion? or is the 990 actually "summing" the bass from the extra channels even though it is the same information as the side surrounds? (they are all set to the same crossover, 80hz).