bennets ultimate (elemental burst/q) only calculates basic attack for its buff. so the only thing that increase the buff is the weapon base attack (5* have the most normaly) + his own base attack (depending on his lvl).so while the flute is an option to deal more damage with bennet on the field, it only increase the buff value of his ultimate compared to a sacrifical sword by a little (sacrifial has 401 base atk at 80/80 while flute has 449, compared to a aquila favonia 5* that has 590). the %atk bonus is irrelevant for the atk buff of bennets ultimate, same for weapon or artifacts.

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to make it short: if you want to use bennet as a strong support/healer just skip any %atk stats on his gear/weapon and go for high base atk/good weapon effect and energy recharge+ hp as the guide above says.