Go talk to them. Introduce yourself. Let them know during this conversation that you're a stereo nerd, but that you're into quality and that loudness is not the name of the game. Give them your number and invite them to call if they are ever bothered at all. And maybe set up hours after which you agree to knock it off.
A conversation is probably worth more than all the soundproofing in the world.
A conversation is probably worth more than all the soundproofing in the world.