Well you have a few options. One is to piggyback your amps. This involves running the input into one amp, then running the output of that amp into the next and so on. This will only work if you amps have outputs; most do.
The second is to get a cross over, eq, or line driver. You might want to send
http://www.mobileaudiocontrol.com/ an email about your application. I think the 3XS supports a single input and allows three outputs, but looking at the diagram it seems that it will only give you front and sub without the rear channel if you use one input. I am sure you can rig it one way or another, soldering iron anyone

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I have always had great success with both of these techniques, you just have to figure out what will suit your application and wallet. If you can give me a few more details about he equipment you have or intent on having I could probably give you more details.
BenjaminS