Then I'd just save the money towards the theater, and build a sub for that day. You're going to want all the play money you can have at your disposal for something like this. That also likely means forgetting about the saw upgrade. According to Bill Fitzmaurice, designer of the Tuba HT, even $100 is enough for a table saw to build your sub. He says so right on his website.
JMO. Otherwise, do as you please. If you have to spend now for a sub, I might consider selling the digm to pool funds into a single sub upgrade for the time being. A thought.