A schematic would be more helpful if you had a scope to follow the signal path. You can check voltages from input to output with a multi-meter. With a scope and meter, you can always use one of the "good" channels as a reference for the voltages and also compare the audio signal readings with the scope.
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sapphire ST2 mkII SBR/SBL <> sapphire SS SR/SL <> Verizon/DVR <> Vizio M70Q7 <> Oppo UDP-203 <> Amazon 4k Fire TV