Haha this can be a difficult fix, especially if you have a lot of furniture or fixtures that will reverberate with the lower frequencies coming out of your subwoofer; so even if your subwoofer is anchored into the floor or otherwise designed to eliminate cabinet resonance, chances are that you're going to have something else resonating that you'll be hard-pressed to do anything about.
Example: one of my speakers is solidly anchored into a concrete wall but despite that if I pump the volume up I have wooden tables that aren't even in the same room vibrating, so I've got those padded as well.
As said by PaulD mass is your friend, and having solid walls help, but that may not be viable for your apartment.
I noticed your subwoofer is ported though? Try stuffing something into it, it can help. I've always preferred sealed cabinets as they tend to have much tighter (less "boomy") bass and that can greatly reduce unwanted resonance in my experience, and if I end up with ported speakers (like my Yamaha NX-N500's) I always shove a bunch of packing foam into the ports to tighten up the bass.
Hope this helps! And good luck!
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