I owned a Klipsch 120SW that I bought new for $300 a few years ago. As I didn't find it particularly musical. It provided some low (29hz) sound, but it wasn't quick or tight. I gave it to my dad to use with his small 2 channel tv setup. Recently, he upgraded the stock power cord to a $90ish Wireworld and uses the Wireworld oasis RCA cable with it. That definitely brought it to life, but I'm not sure I'd still want it for a music only listening system.
Subwoofers add ambience regardless of what your towers can do. I had an interesting coversation with the tech at JL who told me that a subwoofer adds venue ambience even with a vocal only track. He had done blind tests and people preferred the subwoofer on when playing vocals... I found that interesting.
That being said, you want a subwoofer that is quick and articulate for 2 channel listening. Those usually come with a higher price tag....