I agree with the duck.
To me, It has to be a setting of some sort being off. That amp is a proven tank. Time and time again.
Also, don't worry about dropping a volt and not making any power. To the ear, you wouldn't know if you where holding 15v or seeing battery rest. Take a picture of the gain, low pass, and infrasonic dials please, and report back.
Edit- and you paid someone else to turn a knob? Actually, let me re ask that. A asshole at a shop took your money to turn a knob ? That guy is a dick. Not only using his ear, but even charging you to do it. What a stand up car audio lover. Jesus.