What size room are you talking about here?
I would think that your two 12"s would be way more than enough for most any space in a typical house. I'm going to throw out a third option for you, other than plate amps or trying to find a 14.4v supply. You should give some serious consideration to using a decent quality pro power amp, as this will give all the power you're looking for without the hassle of building a plate amp into your box or trying to screw with external power supplies and capacitors. A very reasonably priced unit that has no fans to generate noise is the Behringer A500. It is good for 2x230w at 4ohms and bridges to 500w at 8ohms, and can be found for about $170 new.
Here is a link to a thread on AudioHolics where the amp was subjected to a pretty rigorous test routine. Later on in the thread there is a lot of into about people using these to drive high-end home theaters.