Is a Sub worth the bother?

I have a small, simple system that pretty much meets my listening needs. It's an NAD C715 (all-in-one deal) with a pair of Epos EL3 bookshelf speakers. I've been kicking around the idea of getting a cheap subwoofer with the idea that it would add a little oomph to things. This is strictly a music system, not home theater. I guess my questions are: 1) would a sub help that much or am I just limited because of the EL3s? 2) If a sub would help, would a cheap one be okay (I'm thinking along the lines of the Dayton sub 100, perhaps an Energy Take Classic 8--$100 at Costco these days). My room is roughly 12 x 20, but again, I'm not looking to bring down the house/neighborhood.

Thanks in advance for your thoughts.