Right here goes.....Its my everyday job so should be able to help. (Hope i don't get something wrong)

Passive Crossover - The full range signal is taken from the source (1 input), passed to the amplifer (via RCAs). The amplified signal is then passed to the crossover, where the signals are then split the signal up with respect to the speakers you are running into mulitple outputs, in your case 3. Therefore only use 1 amplifier. These are usually designed so they operate at 2 or 4 ohms when the load of the speakers are calcuated as 1 value.

Active Crossover - Usually seen before the amplifier stage. What happens is the same signal source us split into multiple pre-amplified signals. These signals are then sent to the amplifiers (i.e. if three signals are required, then three 2 channel amplifers are required). Much more expensive but a lot better sound. This then means that each individual speakers impedance needs to be know so as not to overload the amplifier.

Impedance - Speakers operate at different impedances depending on how speakers are produced. Please correct me if i'm wrong, but if you put speakers in a series circuit, i.e. 2 ohm speaker linked to another 2 ohm speaker to impedance doubles and therefore only pulls 4 ohms from the amplifier. If you put them in parallel circuit, then the impedance halfs, increasing the loading on the amplifier thus making it hotter.

What you also need to look at is teh power rating of the amps. All the quotes of 600W are bollox if its not an RMS value. RMS means this is a power at which you are more likely to see. The 600W means the amp might peak at that for about a millisecond. You ideally need to match the power to the speakers and its always better to have a little too much power, as that mean the speaker isn't drawing the power, more a case of supplying.

Right thats enough, i've even bored myself.