Any power amp can drive any cab, as long as the nominal impedance (Ohms) is within an acceptable range. The power handling capability of a cab is something you will see listed on the spec sheet. There is no direct relationship between the impedance and the power a speaker can handle. If your cab is rated to handle 400 Watts, it should be OK with a QSC rmx850 amp, as long as it has an impedance higher than 2 Ohms. Its unusual to find a cab with less than 4 Ohm impedance, so again, its probably OK.