We are upgrading our church sound system and just bought a Behringer Eurodesk SL2442FX-PRO and the Samson SX1200 power amplifier. I'm not sure the specs of our speakers, but we have 2 mains and 3 monitors. We'd like to keep the mains with Left and Right capabilities and have 2 separate signals for the monitors (2 will have the same and the pianist's monitor will be separate). My question is, "Can I do all this will the 2 channels from the power amp or do I need to buy an additional power amp or exchange for one that has 4 channels?".

The SX 1200 supports 4 seperate speakers without problem but you must use 8ohm speakers. The manual covers this setup very clearly. Also ensure your speakers are 300W or better.

If I guess correct your speakers are passive. That means that they have no internal amplifier so they need to take signal from power amplifier. A way to examine that is that your speakers have no power supply input or cable (in reverse the active speakers have power supply input).
Assuming that this thought is correct you can not use your Samson SX1200 power amplifier (2 x 300 watts at 8ω and 2 x 450 watts at 4ω). You asked 2 main channels (left, right) and 2 monitor channels, so using just one stereo power amplifier is impossible for this combination. I suggest you buy 1 more stereo amplifier in order to use it for the 2 monitor channels (assuming that you use Samson for the main speakers). I don't suggest exchange Samson for one with 4 channels because in case of a problem you will have none power amplifier and you will have no sound at all. The power amplifiers also, when are using in high power levels, tends to make heat. Using 2 power amps you can manage this heat easier. I've done these thoughts assuming that all your speakers are in good condition and sound great. Otherwise we can create a new plan replacing some speakers with new ones active technology so you could avoid power amplifier channels.
These are my first thoughts about your case. I'll be glad to hear news or requests for other modifications from yours.

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Thanks and regards
Stelios
direct FixYa link: http://www.fixya.com/users/technical114