The rear channels on that amp are called passive outputs. You have no fader so you have to use attinuation moduals to get the sound you want. It is a good amp and I have one I used in my M3 and and liked it once I got the tunning set the way I liked it.
Its not a hard amp to work with but its also not a novice amp that is easy to tune. You have to buy the moduals to tune it for the passive outs and the crossovers.