You ARE making this far more complicated than need be. A good aftermarket amp has built in, adjustable crossovers so all it needs is basic 2 wire stereo inputs (your rear speakers) and the amp does the rest, no need for more crossovers, etc. All you need is one good amp, one or two subs, a little wiring and that's it. It's easy. Don't even consider the center surround speaker as being any part of your equation nor do you need a 5 channel amp if only to be used for adding subs. Most good aftermarket amps are made to be used for either amplifying several door speakers (4 channel) or can easily be bridged for one or two subs and have selectable crossovers. Each amp comes with detailed instructions that'll explain how to do that either way.