Sounds like its time for an alternator upgrade, the true way to get more current going. You can get it rewound for couple of hundred, or possibly buy another one. As well, look into an Optima battery....
As for having all front volume, that's how it should be. You should hear everything up front, not from the rear. It's called staging. Rear speakers are for the rear passengers and possibly some fill, but it's all about the front speakers. But if your 15" sub isn't loud enough, then you probably have hearing loss so can't help you there...