It's possible there's an issue with the power supply voltages. Voltage regulators can fail and/or no longer regulate properly. Voltage may have dropped causing a decrease in gain. Depending on the circuit, that would account for both channels being down in level as they share a common voltage regulator. It's also possible a fuse or resistor in one of the power supply rails has failed. This can also cause a problem with the power supply voltage. If this is a tube unit, a tube that's common to both channels may have failed, usually the input tube.
Help diagnose problem with my amp
I have been using a Myryad SS integrated amp (MI-120, 60 watts/channel) in my system with ProAc D15's for a few years now. I have been plugging the amp into an Ensemble Isolink then to the wall. Last week, the volume has dropped by a quarter of the dial! I checked all connections etc. and nothing appears amiss. It's not just one speaker, both speakers now play at a far softer volume at 9 o'clock on the dial, which previously was plenty loud and detailed. I now have to turn the volume up to 12 o'clock for acceptable listening levels of depth, body and detail. This morning I wondered if somehow the Isolink had begun muting the amp's current, so I unplugged the amp from it and ran the power cord straight to the wall but no change. Any ideas?