Only 2 things stop bass from reaching your neighbors - mass and sealed air.
Mass means materials dense enough to inhibit the very long waves that low frequencies produce. In practical terms, this means thick lead or vinyl sheeting, bricks, stones, concrete, etc. Mattresses, foam, egg cartons, posters, hopes and dreams, etc are not going to be effective. Gypsum board is an economical compromise. BUilding a brick wall would be more effective, but mroe complicated than screwing on additional panels.
Sealed air. Means just that. New windows have gas sealed between multiple panes, and the all the fram parts fit tighter. Building additional walls seals a section of air inside. The idea is that there is nowhere for the air to be displaced when the sound wave attempts to go through, thus providing resistance. Taping a bunch of balloons to the wall will not work because you need just a very small breach in a barrier for effectiveness to drop.
Most likely, you are sh!t out of luck. I've lived in a few places where we could do parties, but these were always isolated houses or in industrial buildings, and even then we had rules and limits we needed to follow. If it is a matter of studio work, continuously worki with loud volume is jsut going to fck up your hearing anyway
Short of making a room within a room... it's impossible.
A better plan would be to develop one of those CIA sound lasers, point it continously at your neighbor each time they enter or leave their home. Maybe you can deafen them over time.
