Yes, you need a preamp. No instrument that I can think of puts out enough signal to drive a power amp on its own. I use an RMI Basswitch to drive my QSC GX7. It sounds FANTASTIC! In the past, I've used a Michael Pope Preamp, an Aguilar Tone Hammer pedal, and a Sansamp BDDI to drive QSC amps(which always sound great!), Crown Amps(which almost always sound great.), and I even tried a Behringer Europower amp(which we won't talk about).
The main thing is you want to find a pre that has enough output to drive a power amp. The good thing about QSC amps is they have a slightly lower sensitivity rating than some other amps, like Crowns. I think pretty much any rackmount pre will work. If you want to use a pedal(Basswitch, Tone Hammer, Enabler, Sansamp, etc), contact the manufacturer and make sure it has enough output to drive a power amp.
Fender MIA Deluxe Jazz Bass V /MusicMan StingRay 5HH/Custom Moye 5/RMI Basswitch/QSC GX7/LDS Cabs