Alright start here, FOR example, to get the most out of this, get a 4-channel amp, I wouldn't go used as amps age just like cars do only much quicker, one thats been beaten on for a year or two is gonna perform worse than a out-of-the-box amp.
We'll use this as an example, its imo the cheapest 4-channel your gonna find for the wattage.
http://www.pioneerelectronics.com/PU...eries/GM-6300F
Its a 4-channel bridgeable 600w amplifier. 150w per channel bridged at 4ohm this is what we're going to go for, however, we're gonna do it at 4ohm and 2ohm, the difference here is your speakers will have more wattage put to them at 2ohm but will distort easier at higher volumes, I wouldn't worry about overheating it unless you put it in a confined space. I run an all-around pioneer setup myself amplified and have never had a problem with GM series amplifiers.
Now you can run all 4 speakers at 60w x4 at 4ohms it says, thats 1 speaker per channel (4 channels). But this is a waste in my opinion because your only feeding 10 more wattage than the average headunit, but your getting cleaner clearer sound. So lets parallel wire the speakers.
To parallel wire, you join the positives of both speakers together, into a Y, and then run the single end to the + on one sides bridge, do the same to the speakers negative to the - on the same sides bridge.
What you did was lowered the speakers natural impedance from 4 to 2, and wired them in a bridge on one side in place of wiring all 4 wires into that side, you now only have 2, and your speakers now pull 150w a piece, instead of 60w. Do the same for your front speakers to the other side.
I would only bridge equal wattage speakers, otherwise you can severely damage them. Ive bridged 1,200 and 1,500 watt amplifiers before running 6x9's because you can always turn them down. Its always better to overpower than under power.
I would say you could do this with a 2-channel amplifier, which most morons would try, but wiring 4 speakers into a 2-channel would create a 1ohm load i believe which would send all the wattage from the amplifier at all times causing it to overheat and fry. Usually only mono blocks are made to run at 1ohm.
http://www.caraudiobook.com/ohms_law/ohms_law.htm
The first picture is parallel wiring. But like I said, it can distort at higher volumes a little bit easier at a 2ohm load, but it shouldn't be a problem running 150w through a 6x9 or 4x6, its about the right amount. You don't want to series them, or you'll be down to 30w a piece. GL
