designing a crossover

Hi. First post here. I am trying to design a crossover/mixer/low pass filter for a friend of mine. He has 2 amps (Fusion, about 2000W, not sure the model), with one sub per amp. His current setup runs RCA from the head unit to the input of one amp, and another RCA cable from the out jacks of amp 1 to the input of amp 2. I would like to build a box that will sit between the head and the amps, so that the RCA runs from the head to the new box, and then a separate RCA cable runs independently to each amp. I am also trying to design a variable low-pass filter into the box. Not sure if I want a separate filter on each line, or just one filter in the circuit before the signals are split. Also, a basic gain function would be desirable, perhaps using an op amp or nice MOSFET. I have a decent understanding of electronics and can assemble PCBs, have a stock of standard parts, etc. Any help would be appreciated. I'm especially interested in knowing how much signal current/voltage typically runs out of a head unit to the amp, and what the impedance of a typical car amp is. So, I'm looking for designs or ideas for: a preamp a low pass filter a splitter circuit (opposite of a mixer, not sure what you call that) Thanks!

You need to meet Rod Elliot.  Here's a project that you might find interesting.

http://sound.westhost.com/project08.htm

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Rod's a great suggestion. I was going there, too, till DYohn beat me to it! :) Also check out Phil Marchand's stuff... www.marchandelec.com