Try moving the Sub to a different place in the room. Bass is fairly non directional, so if you move it from the wall that is between you and your neighbor, there would be likely less transmission.

Is the Arc mounted, or does it sit on something? 

The townhomes I used to deal with in Palo Alto had a rule about not allowing mounting of speakers to shared walls, for this exact reason. 

Hi @Wonka458.

Welcome, thank you for reaching out to Sonos Community.

Does any of your speakers especially the Sonos Arc is mounted on the wall how about the sub is it right beside your wall?

I'm not too sure but it might be the Sonos arc since it has a 3D effect sounds with eleven high-performance drivers for crisp highs, dynamic midranges, and surprising bass

Have you also tried to adjust the E.Q for the soundbar, If not you can check this article: Adjust the bass, treble, balance, and loudness

Let us know if it works. If you need help with any other information, please be sure to let us know.

You could try something like this.  Never tried it myself and I am not recommending that particular product (some reviews don’t look very good), but I think you get the general idea.