Ive had the same issue...with down stairs neighbours knocking my door once. I've added speaker isolation, and that completely stopped the knocks. Bass really travels through the floor. Maybe you can try adding svs sound path isolators to your sub
I assume your apartment has concrete flooring?
Is it a high rise apartment or small one ? ( asking because for high rise apartment I noticed noise travel is lot less, may be because of heavy concrete)
You also used something like a SVS sound isolation path?
You have any other dampening material around the subwoofer? (carpet or acoustic walls nearby or at corners?)