The question I'm left with, is there something that needs improving or are you just looking to spend money?
Perhaps supporting musical education or other charitable work to support culture is a better spend for you right now.
I mean, yes, you can get more dynamic range with another sub, or larger surrounds, but you can only listen so loudly, and distortion can only be lowered so much. If you make it your life's ambition to improve your hearing to the point where you hear every 0.001 of distortion you'll never be happy. :)
Otherwise, movie posters, a popcorn kiosk, books on old movies.... :) all sorts of other ways.
Best,
Erik