All three of the same type SMD capacitor had gone bad.
I recently had a similar issue with a D-9, one channel was 1/10th the volume of the other. Armed with a scope and the schematic, I tracked it down as well to a SMD electrolytic coupling cap between two stages. Curiously, this player uses a SMD electrolytic for one channel, but a through hole type for the other. (one of the reasons you don't stand much of a chance without a service manual, keeping track of the signal path is a nightmare - the signal enters and leaves the same chip several times, goes to another chip, comes back, changes PCB sides, goes to one single cap on the other end of the board, returns...)