I would bet that the OPPO clearly outperforms the Outlaw for processing. My BDP-83 was better when connected to the 990. It can't hurt to listen both ways and see which one you like best but make sure the levels are matched when doing the comparison or the louder one will win.

All your lossless/SACD/DVD-A decoding should be done by the OPPO and it is probably better at the lossy stuff too. I think you are correct that the 990 will process the LFE and apply bass management to it. You may have to tell the 990 your speakers are small to get it to properly output the LFE signal. If they are set to large the 990 assumes they should get the LFE. There is also a L+R+sub setting for a double bass type effect in which the Mains are set to large and the sub is added in also.