First of all, are you certain the power rating has decreased over time? Have you had the same measurement when you first had the amp? Like someone else mentioned, it's within 5% of the rating. Maybe your amp was assembled that way.
Power rating is not like the maximum speed of the car where you measure the speed with full throttle. Power rating is measured *as relative to the distortion*. As the power goes up the distortion goes up after certain point. Are you sure you are using the same distortion percentage for clipping as published spec?
Power rating is not like the maximum speed of the car where you measure the speed with full throttle. Power rating is measured *as relative to the distortion*. As the power goes up the distortion goes up after certain point. Are you sure you are using the same distortion percentage for clipping as published spec?