The correct, or even optimum, wire gauge is not about something problematic when applied to speaker wire, unlike amplifier supply wire for example. But as you increase the wattage you increase the current and this impacts the resistance. Resistance directly relates to wasted or lost power. reistance is a factor of both current and distance. So after all the investment made in amplification and tower speakers, you do not want to take a short cut on wire. Instead, you want to ensure that your investment is not compromised via some other link in the audio chain. Another way to view it is that most everyone uses a heavy gauge wire to their 500 watt subwoofer. You should apply that same gauge of wire if you are running 500 watts up each side of the tower.
14/4 is perfectly fine for 200 watts per channel times 25 or less feet.