Ground could be a HUGE possibility. Along with the 8 ga power wire.
That amp will only require 4 ga, I would run 4 ga from batt to amp and amp to gnd, but that 0 ga for the gnd won't hurt, in fact it'll be overkill, and overkill is better than okay.
Make sure it is grounded well. No seat bolts or any of that shit, measure, drill a hole and sand the paint around the hole you drilled down to bare metal. Most say you should go directly to the frame, I didn't, mine is in the cab of the truck and my ground is fine.
Your gain is probably fine, but for the moment (until you can redo your power and ground with the proper gauge wire and do a good ground) you might want to turn the gain down a bit to avoid clipping.
Start with that, I would bet money that is your problem.