Best Buy review: Amplifier installation

I had the geek squad install an amplifier in my GMC Sierra with a Bose system. The $3000 JL Audio subwoofer system I have sounds terrible. My subwoofer is receiving all the wrong frequencies. Initially the tech told me it was my amplifier because he had never heard of the brand name FOCAL. Okay, so I returned the amp and purchased a JL Audio amp. The system still sounds terrible. The ground wire the tech installed is half hazard. No paint was cleared from the metal before adding the ground and he used self tapping screw that's looks to be about the size of a 22 GA wire. 1200 watts is not getting thru that tiny screw. Secondly I suspect he connected the (LOC) line out converter to the wires off the door speakers which again is incorrect. The signal from the wires at the door speakers are after the Bose amp has already altered the signal for what best suits the door speakers. Meaning my subwoofer is not getting the correct frequencies to create the correct bass lines. He should have either attached the LOC to the full range signal from the stock deck or from the subwoofer signal coming off the Bose amp. This tech was not trained properly at all. Ground wire attachment is elementary and should be one of the first things they learn. I went to Audio Express and had to pay more money to have them fix the ground wire connection.