OK, let’s revisit that idea! Maybe the old power trace was already a strong indicator of a failing subwoofer and all that random noise and high power-draw (even in standby!) was an early omen that I simply did not recognize at the time.
If we look at the numbers, maybe there was an extra 9W power leakage due to the amplifier already going bad fast.
If we remove that offset and noise, the two amplifiers are quite similar (~0W standby, ~6W use) and the 9W was additional and abnormal.
My knowledge of electronics is not sufficient to be sure about the exact mechanism, but the inner glass fuse of the old amp was actually blown, indicating that the current got high enough to take it out.