Some REF40s are susceptible to large mains power spikes and decrease the volume setting without any change in the display after such transient.
I found it once when comparing power amplifiers - sometimes the differences were larger than expected after switching one of the amplifiers on, although we apparently corrected the volume for equivalent power. Just turning the volume step by step to 0 and increasing from would reset the scale.
Powering the amplifiers from a different circuit also solved the problem.
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