Those subwoofers are good for a budget. 400 watts RMS of total bass is going to be loud enough, especially if you've upgraded your speakers and are relying on your deck to power them - Your deck is most likely going to provide 15-35 watts RMS to each speaker. A Ford Escort's trunk is not well insulated, meaning 99.9% of the bass is going to pass through to the front of the car. So, if you put louder, more powerful subwoofers in your car, you're really going to have to consider upgrading your speakers again and replacing them with speakers that handle a minmum of 50 watts RMS, each, and also adding a 4-channel amplifier to power them, otherwise, your bass is going to "muffle" out the speaker's detial (highs and mids).
The most important part of a system is having a well-balanced system. You don't want too much bass or too high of highs. Everything needs to flow well with each other. It's like going to the gym - if you put all of your effort into your biceps, your biceps are going to be big but everything else is going to be... You should've put effort into everything else, too. Know what I mean?
The Class D Kenwood amplifier is a better choice. It's powerful, more efficient and puts out cleaner watts. Wire the subwoofers in parallel to get 500 watts RMS from the amplifier. Be sure to turn the gain control on the amplifier down or each subwoofer will be receiving roughly 50 watts RMS to much.
The power used from the amplifier shouldn't cause any problems for your car's electrical system to have to replace the alternator. A capacitor is the last thing you want to turn to in order to provide extra power to an amplifier during it's peak moments. The most you'll notice from a capacitor is that it wil prevent your headlights from dimming so hard, at night.
