A lot depends on finding out, what's on the circuit.
It's highly possible that a circuit in your house feeds more than one room. So if you have your amp, TV, sub, all on the same circuit; (not necessarily the same power strip) it's entirely possible that load is tripping the breaker.
Does the circuit trip during bass heavy passages? That's when the sub presents it's greatest load.
Where is your subwoofer plugged into?
For a quick test, unplug the sub and play same movie, or Wii game that tripped the breaker last time.
Try that before the next step.
The next thing would be to find out how many rooms are on that circuit, and what's plugged in.
First, get a test lamp; you can use a desk lamp. Shut off the offending circuit, and go around with the lamp and see what circuits are dead. For example, if you find out that a computer, or water cooler is on the same circuit along with your TV and sub; that is more than enough to trip a circuit.