i always gauge the proper gauge (pun not intended) by the fuse rating of the amp. wattage outputs can be underrated or overrated, RMS or peak, and you never reall know what kind of current the amp will draw unless you look at the fuse. remember that maximum power (wattage) output is volts X amps. if your amp has a 30A fuse, then the max output will be 30A x 14V = 420W.
so, use that equation to determine the gauge wire. also take into consideration length, as resistance is length dependent. in general, i would say for a 30A fused amp with less than 15 feet of power wire, 8 gauge is fine. now, for every doubling of that fuse rating (or if you have two amps, 30A each), reduce the gauge by half. so for a 60A fuse, or two 30A fuse amps, run 4 gauge. this equation will work for most applications!