Some songs use the stereo effect to a greater degree than others. Put on some Jimi Hendrix for example - some of the songs I have use stereo a lot, and by incorrectly running mono, you would not hear some things at all. By running mono correctly, you'd hear them, but not as intended by the recording artist. Seems like you could use that amps built in crossover, but personally, I steer clear of Behringer amps, as I haven't heard amazing things of them. I've also heard that their listed ratings are very generous to what the amps can actually handle. The limitation of the amp's inbuilt crossover is you can't fine tune it to what your speakers require, it is a simple high or low pass at a set frequency. Let me put it this way: if I were going to run my whole PA off one amp, I'd be looking at a QSC or Crown amplifier, possibly Australian Monitor, and it would be a big mother. As I said, its an important piece that you don't want to fail on you!