Low volume from SB Duet Receiver

I have had a SB Duet system operating fairly reliably for a few years now.
The receiver plays through a Rega DAC.

Suddenly yesterday, the volume from the DAC became very low...I have to turn the amp up to full blast to hear it at all.
I also have a CD player playing through the same DAC, and it still sounds normal.
If I swap input ports on the DAC between the CD player and SB, the CD still sounds right, and SB still very faint.
...so it apparently is not a problem with the DAC.

Powering the SB off and on didn't help.

I am prepared to believe the SB is toast, but from my limited knowledge of digital sound it seems odd that the digital signal is still arriving at the DAC, but with the volume turned really low.

Any insight or advice would be appreciated.