The crackle ended up being a faulty TRS cable, between the BT unit and the car head unit. How embarrassing!

I had no luck getting the VMA2016 to actually work. There conflicting wiring diagrams going around on the internet; some showing earth in different spots, and unclear information on the use of the EN (enable) pin. So it's in the parts bin for the moment. Might get a preassembled VMA2016 board to see if that will help (original issue was low sound volume).

Anyway, I rebuilt the whole unit to neaten it up, and put the power on a seperate circuit. The LM317T was getting hot (burnt me), so I fitted a makeshift heatsink, which brought it down to 69.5C maximum with the box open (measured on the body of the LM317T). Also drilled some vents into the top of the box to aid with cooling. If it becomes an issue, I might look into mounting a small fan in the side of the box.

Pictured attached for dr_frost_dk!