There is a significant benefit to hooking up the optical. The 990's DSP section and DAC's are better than the OPPO's - using the optical connection for DVD-Video will give you access to Pro Logic IIx decoding (good for getting surround sound from Dolby 2.0 sources and getting 7.1 from Dolby 2.0 or 5.1 sources if you have surround back speakers) as well as Dolby EX and DTS ES decoding, and it will bypass the OPPO's rather lackluster DAC's.