Yes, get a 2nd of identical model. This will give you about 2x the output, and give you better sound quality in the room if you put each in a different section of the room, measuring and finding the place that averages the response out as smooth as possible. Depending on your crossover frequency(if it's like 70 Hz or above especially), you would gain substantial sound quality by placing each of two matching subs within 3 or so feet of each main L and R channel, resulting in better integration of transition points from mains to subs. Employ a device like the Behringer DCX2496 into the mix, and you can definitely get optimal response with multiple subs. But that is an extra expense and ideally you should use an outboard amp for the mains so the DCX can handle both the main and sub xover and correction filters. It's a far more sophisticated/powerful device than any receiver's internal xover/filter system.
-Chris
