In order to achieve a 4 ohm setup you would have to use only one voice coil per sub. Typically, each voicecoil is made to handle half of the rated power (so 250wrms per coil). You run a very high risk IMO of frying a coil. Go with a 2 ohm mono setup (4 ohm per sub wired in parallel).
I have a 300wrms class A amplifier, 2.5 farad cap, and an alternator rewire (made a HUGE difference). My idle voltage sits at 14.5v but the lights dim a tad when I bump it. If you are concerned about the amount of juice you are going to be drawing, then go with a class D amplifier. Class A is about 60-70% efficient where as class D is around 90%.
