If you can reach, I would try power cycling the Denon first by unplugging it for half a minute and plugging it back in. I've solved countless electronic issues just by restarting them.

My Onkyo AVR has a feature called "Intellivolume" that allows me to adjust the gain of the individual inputs, whether RCA, optical, or HDMI. Perhaps the Denon has a similar feature that isn't at its default settings.

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