Honest answer, you could go up to their apartment, use something like AudioTool for Android, and figure out what frequencies they are hearing the most of, then EQ that to be lower.
Unfortunately there’s not much you can do to stop transmission in an apartment, but what you CAN do sometimes, if you are lucky, is balance your bass using bass traps and EQ and hope that the same notes that were too much in your apartment are the same that are too much in theirs. Talk to GIK about their soffit traps. I want to downplay this solution though, it will make your stereo sound better, but it's a matter of trial and error as to how much it will benefit your neighbor.