I would never give up my pair of Vandersteen 2Wqs. These subwoofers blend seemlessly with my mains (now Ohms, but previously they blended equally well with my Vandersteen mains). Check my review of these in the review section. Properly executed and set up, a dedicated, powered subwoofer can do things that most full range speakers simply cannot. I suggest you read about them at the Vandersteen web site. Their design is somewhat unique and addresses two major issues of subwoofers: crossover and placement. I got mine used here on Audiogon, but even new, they are a fantastic value for a music-oriented sub (as opposed to home theater), IMHO.
Sub Question
I tried to do a search, but could not drill down on this. Anyway, I have never owned a sub. I never really felt the need. However, I had a friend tell me that a sub does more than just enhance bass output and perhaps bass quality. He went on to say that having a sub takes pressure off the driver thus making the speaker do less work. Because of this, the speakers will sound better or have better clarity. Thoughts?