Quick update. I wonder if it may be heat related.

After having the processor off for about an hour and returning to it the sound level remained stable for about 20 minutes after which the sound level increased again until muted or powered off.

I do have the processor in a rack on the Meridian rack shelf with fans operating and a cool ambient temperature.

558.2 amp, Meridian A500 speakers, M2500 sub, Director & Explorer DACs