Did they do this? This "bracket" is silly. The problem I have is they were willing to drill through your metal for that, but not to give you a good ground.
Your ground ends at your chassis. From your amp, it is going the path of least resistance and ending at your chassis. If you have better access to your chassis, then your grounding cable should be longer and go there.
From where that it is, your ground goes through that bolt, through sheet meal, and unless that's a unibody car, it is going through a grounding strap to your chassis.
If you want to fix it yourself, (or tell them how to do it). That grounding bolt looks raised off the sheet metal. It looks like you are getting half the contact you would if, you would just drill a hole through the sheet metal, clean it, and ground anywhere else flat, that the connector sits flush against the sheet metal.
I still think your ground loop is being caused by all those cables being ran together.
P.S. Tell that rookie to cut his zip ties flush.
