Is 14 Gauge an good speaker wire size?

Use 8-10 gauge wire. I have friends who have fried their amps due to resistants between the amp and the subwoofer. With a bigger gauged wire it's more capable of holding more power. even with a smaller system like yours its a better idea to stick to gauges like that. and if your subs are 400 watts rms i would shoot for as close as possible to 400 watts.