With those speakers, adding a second sub like the one you have would allow you to configure them for stereo bass with the least expense.
To clarify my earlier post a little, just because I prefer like with like doesn't mean others feel the same. There are plenty who prefer OB bass regardless of what it is paired with. And, when it comes down to comparing just the bass, I prefer OB bass hands down. However, when considering the entire spectrum, I find the overall experience to be more pleasing when the characteristics of the bass, midrange, and treble are the same.