I just connected the subwoofer multichannel output line directly to my sub from the oppo and it did receive signal. The problem has to be either the way I have the 990 set up or a maybe a defect in the 990.
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Marantz AV7702 mkII
Outlaw Model 7125, Outlaw Model 2200 (3)
LG OLED C8
Oppo BDP-83, LG UBK90 4k
Directv, Sony PS4 Pro/PS3
Polk Audio RT800, CS400, FX500
SVS Elevation (4)
Sunfire True Subwoofer, Epik Empire
APC H15
Harmony Elite remote
Blue Jeans and Monoprice interconnects