I've used both QSC (K and K.2 series) and EV PA self-powered speakers fed by a preamp.
QSC goes so far as to say that they have an optimized DSP program on their K.2 series for bass players, however I wouldn't say they're particularly good. Placing a preamp before the amplifier does help, but those speakers use a lot of multi-band compression and dynamic EQ which will begin to fight you at Rock-Band levels. You can't override some of that compression or dynamic EQ on those boxes because it's there to keep you from destroying the speakers.
Out of all the people I know who tried the QSC K.2 solution, the only way some of them got it to work, was to team it up with one of the QSC sub-woofers - but by then you've obliterated the weight savings and compactness of the rig.
There's a couple of fusion guys who are using custom Meyer Sound integrated systems as bass rigs. It's pretty funny to see a tiny inverted J-Array up on stage just for the bass player.
Honestly, You'd probably be better off seeking out a powered bass-specific cabinet like the Phil Jones or Bag End products.