thickness of ground cable should be same as the power cable youre using. usually we prefer grounding cable to be short as possible and we also prefer soldering instead of bolting. also best to use a grinder to get the paint off until bare metal chassis.
when we say earthing problems it might not just be for the amps, bad rca cables or solder points can also give same problem, anything that has +ve and -ve means power & earth also. so other equipment can also contribute to the noise.