Dear Cuda,
I would recommend STF-3 for rooms up to around 4500 ft^3.
Definitely a square room is not ideal for reproducing bass. Also, it is best not to sit in the middle of the room, so if possible try to avoid that.
That said, you should be able to get satisfactory results. In the future, you could even try using the STF-3 in the front left corner to handle deep bass, with our mid-bass module placed nearfield to handle the mid/upper bass. For a $50 fee at any point in the future, if you wish we can modify the STF-3 amplifier to accomodate fixed extended bass mode (one port plugged) instead of maximum output mode, where you would have strong response in the 16-18Hz range while giving up some maximum output capability above 25Hz. Another alternative is simply ordering a port plug for $15 shipped, which will result in an overdamped response, but will give stronger output and response at 20Hz.
Sincerely,