I personally would go with 4ga wiring to be on the safe side of things. Looking at the specs of the sub, I would say yes you are underpowering it. Here is what the Kicker site rates your 10" sub, 1200 Watts Peak and 600 RMS. Your amp does, 1 channel(s) , 300 Watt - 2 Ohm - THD 1.0 % - 12.5V DC - 1 channel(s) , 210 Watt - 4 Ohm - 14.4V DC - 1 channel(s) , 420 Watt - 2 Ohm - 14.4V DC - 1 channel. So even at its peak it is still NOT getting the max RMS. Its almost 200RMS below its rated level.
So in sort, YES you are under powering that sub.
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