I recently bought a QSC K12 to use as an on-stage monitor for my keyboard rig. I play bass primarily, but also do some keyboard stuff and I have been lugging around 2 Ampeg SVT-810e cabs. I didn't expect it to keep up with an 810, but after reading some online reviews and a live demo at Guitar Center, I gave it a try. With just a bass amp (Ampeg SVT-CL through an ampeg 810), keys thru the K12 and drummer in my practice environment, it wasn't loud enough. Not even close. I had to run the keys though the K12 and the PA in order to hear it. We don't play extremely loud, but it is rock and roll.
I debated whether to keep it (I got a good deal on it at $656) and tried it as a PA monitor. It worked well for that application. FWIW, I didn't try my bass though it.
I ended up returning it.
--jack