Let me toss the ball back into your court. Assuming this effect is real,
what can you tell us about it and what measurements have you made? Was it
accompanied by a change in the output voltage from the amplifier's power
supply? How long had the amplifier gone unused before you did the
experiment, and could it be improving simply because the electrolytic
capacitors are re-forming after a long period of disuse? What was the
nature of the change in the audio output -- have you measured distortion,
frequency response, etc.? Have you compared old and new fuses with the same
electrical ratings? Can anyone other than you hear the difference?