Know What You're Getting
For the guys who are dissing the KSub, you should have known what your needs were. The KSub is not meant for major shows, but for small to medium events & rooms, and it does an incredible job. I actually prefer the punch of the 12" dual speakers with country / rock and smaller r&b / jazz settings. The clarity is ridiculous when paired with K8 /10/12's. Nobody needs an 18" sub unless you're playing bigger shows & events or hip-hop / rap, or if you just simply want more sub-harmonic extension. (I'm sure I'll end up buying KW181s in the future for bigger shows.)
If you're doing a 4-5 piece band and have good mains, this sub will provide good punch and low end extension on kick drum and bass guitar. Obviously, consider your needs before purchasing a sub that you think is "not enough". I have used this sub in 250-300 capacity venues and wedding events with my K12's and the power is more than enough for my needs, and the clarity simply tickles my ears every time. As a matter of fact, I used it last night to DJ a small wedding reception, and the clarity was just ridiculously good. My business partner even commented how "modern" the classic songs sounded (Kool & The Gang, EWF, Stevie Wonder). I also play a small-ish bar & grill venue in a college town with in-house PA, and this is the only sub they have, and the only one they need.
For you who don't really understand how to set up a good sound system, here are a couple of things to consider: Needs.., what is the average size crowd you will play to? Clarity.., harmonic clarity actually carries further and is louder when your system is "tuned" properly. Music.., will you be performing dance music or live music? If this is for a band in a smaller setting, then one or two KSubs would be plenty. If you're doing DJ work for more than 200 people and want that low-end extension that only an 18" speaker can provide, then go with an 18" subwoofer.
In closing, I have two QSC K12's for my local gigs, and the quality and sound are amazing on them when coupled with the KSub. I plan to buy another KSub in the future just to have a pair for my K12's. Love my QSC gear.