Hey @dpop
While you are correct that Watts = V * A, the way linear amps work (with the exception of Sanders' Magtech) is that the rail voltages WILL sag if the line voltage sags. There's no adjustment feature without a voltage regulator such as a Furman with an AVR feature.
Does this sagging matter? Well, possibly. I think this would be a good case study for Amir to do, to compare output linearity with input AC voltage on a linear amp. Due to the way current amplification works, and moderate amounts of feedback we normally pretend it doesn't.