Here is my take in this topic. Their is nothing wrong with one sub in a system if you implement it properly if you have the right resources. Yes dual, quad, etc, no matter how many subs you put in a room is going to improve the smoothness of the response but doing it without the use of tools and listening it not going to solve by itself especially in a room that is untreated. I have a small room where my system is and really might not require a sub but I have it for use of my listening preference, type of music, etc. My sub is set low 50s with the phase inverted. I dont have the sub in the center because I have the component rack in the area. So the sub is off center to the left about 1/4 wavelength of 1/2 of the room node, in this case around 60hz. This is almost 1/3 octave above the sub crossover frequency. The sub is low enough that localization is less noticeable and easier manageable.
REW is the norm for measurement software for this situation. I personal use Rational Acoustics for measurement in the outfield but since I have it available, it makes it easier to see what your response look like. If you are inclined with using measuring software, more power to you. Of course, you ears are the final judgement.