Many causes, and certainly the listener
I agree with all these--
Mood: I've given up liquor, but when I drank Scotch or Armagnac while listening, there was less variation in my response from day to day. Now I don't take the edge off, and the edge can affect things.
Energy: If I'm tired, it's hard to react with the same pleasure.
Ears: As I've gotten older, one ear or the other (or both) is more likely to be slightly congested. The ENT says, "Live with it."
Source material: There are huge differences among recordings, and I for one don't like to listen to the same thing every day.
As to AC power, many say it varies and has an impact. I've never tested or noticed it, so who am I to say?
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