The 975 7.1 outputs are to feed to an amp and a subwoofer. The 990 has those and also 7.1 inputs which are analog. I had my OPPO connected to the 990 via the analog inputs because the 990 does not have HDMI. In your case you would connect the OPPO via HDMI to the 975 using one of its inputs probably BD. Then you connect the 975 outputs to the input of your amplifier(s) for as many channels you are using (6.1).
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Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner

TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv


Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner