If you use a passive crossover network, you should be able to run your two front speakers and your sub off of one two channel amplifier as long as your front speakers and the subwoofer don't see the same frequencies. Where ever the two speakers see the same frequency, your impedance will drop down to 2 ohms. In fact, if your amplifier is stable enough, you might be able to run all four speakers and your sub from the one amp. Just wire in a resistor in series with the rear speakers to keep the sound coming more from the front. The reason I suggest hooking up your amp to your front speakers and the sub is because you will get better sound staging that way. Who goes to a concert and sits facing away from the stage anyway. You want most of your sound to come from the front and in your face not from behind. You won't be able to use the sub outs on your eq unless you use your eq to run your front and rear speakers and use the sub out to go to the amp and then on to your sub. Basically your amp will be dedicated to only your sub then.