When a person hits that subscribe button they are saying, I want this person's content... not I want you @Youtube to decide what portion of content I get.... C'mon guys, to subscribe is an action, which means that sub is saying, give me this content
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Make it a general setting that someone needs to turn on if they want less notification spam. Do not make everyone suffer, generally if people hit subscribe they want to see every video; I for one never have the bell on.