18 awg is not enough for a power amp, I would expect a minimum of 14 awg. At high load the added resistance of the smaller power cord will cause a drop in voltage to the power supply, requiring it to draw more current and thus generate more heat. The amp's protection should shut it down before immediate damage is done, but running warmer over time won't do it any favors.
Same here I would feel a lot better/safer if you were to use a 14 AWG power cord with your XPA-3. How much the amp will be worked depends on your speakers, how loud to you want to go, etc etc. I say get a 14AWG power cord and not worry. If you are in the NYC/Long Island area I can give you one. I have a bunch laying around.
2 Channel: Work in Progress...
HT: Oppo BDP-83, PS3->Outlaw Audio Model 990->Emotiva UPA-7->BJC 10 White