What are the amps? Who is the shop? And did you get the amp from that shop?
Is the amp really on? It's possible the amp is shutting down but the led is getting 12 volts from somewhere on the board. Leaky cap or something. I know most amps have power indicators that tap off the remote lead. Meaning even if the amp is blown the power indicator will still come on or go off with 12v at the remote lead.
It's either that, or as FordTough described. There is something shorting across the power terminals inside.
It definitely has nothing to do with the voltage regulator

Don't let them screw you around. Bottom line is there is a problem with the amp and/or the install. Forget about the rest of the system. No outside influence would cause that to happen.
Make them take care of it. I could give you a few things to try but you shouldn't have to figure it out when you've paid a 'professional' to do this! I'm a professional and I wouldn't think twice about servicing or replacing the amp. The voltage regulator excuse is total BS! I will refrain from ranting on about BS shops and how they make us all look bad.