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?