Add line-out feature/jack allowing connection to amp's line in

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I actually dont use that cord specifically, I ahve one that gots from 3.5mm to 1/4 that I can plug in to my marshal stacks. BUT the rule of thumb I was always taught was to set the volume on the player between 50-75% and then adjust the amp after that. A line out would be nice but it proably wont happen in the way you are asking, having an adapter the converts the headphone jack to a flat line out would be the closest thing I could see.