Since your room isn't very large and you don't "blast anything very loud", the PSW10 may work well for you. But it isn't a great sub, and it will hit its limits pretty quickly. Also, note that you shouldn't be paying more than $99 for that sub (plus any applicable taxes).
Given the small size of your main speakers, the crossover will be set fairly high, so you may find that bass is "localize-able" to the location of the sub (rather than being "omni-present").
Not sure what your budget for speakers is, but you might want to consider a pair of larger bookshelf mains, which will let you lower the crossover and decrease the likelihood of "localize-able" bass.
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