People in car audio shops are still people, and people make mistakes. I agree that professional installation greatly lowers the possibility of a bad ground, but I would caution that it doesn't eliminate it.
If you have a voltmeter, check the voltage at the power connections on the amp when it's working hard. If the voltage drops below 12 volts, the problem is probably with your electrical. (alt, battery, cables, connections, ground etc.)
If the voltage stays comfortably above 12.5 volts at the amp and it's overheating, the problem is most likely in the amp, the sub(s) or the connection between them.
(amp is prime suspect if voltage stays good under heavy load)