Bridge the rear channels to a sub and if you think you need rear-fill, use the passive outputs on the amp. It can take it! If you search long enough, you can find all of the accessories to tune to your liking.
Why don't you go active and use all four channels? Most comp sets will sound much better active. If you want to stay passive then bridge your amp so that you have more power because the crossovers are taking some of the power.