Under powering a subwoofer can cause the voice coils to separate.
Typically when you pick an amp for a speaker you get a amp that can deliver a continuous power to the speakers max requirement so for example your amp needs to output 350 watts continuous power not 400 max. I suspect you are under powering the speaker and that is why it only makes sound if the bass boost is up. Is 350 watts the minimum power requirement for the speaker or the max power? Is the amp 400 watts peak or 400 continuous output? You can change the bass boost if you want.
