here's where the amp is grounded (same under passenger seat for sub amp)
and here's the best shot I could get of the sandwiched lug
so it's basically the seat rail, then a washer, the ground, another washer, and then bare metal. you can barely see the lug going forward in the pic.
as for setting the gain how do i match it to my HU? i have an alpine cda 9886 and it has 4 volt preouts. as you can see in the pic of my gain adjustments it has min, nom, .5v, and max. all the scratches are from me adjusting without removing the seat.