on your chosen earth point make sure it is bare metal it is in contact with, you can paint it after to protect it. Also check your battery to chassis earth and engine to chassis earth
are good and not corroded. Upgrade the wires to higher spec if need be same size as your amp power cable. that should sort your earth if you then upgrade your alternator cable you have then done the big 3, and should be set for any system. But it does seem that your amp psu has become sensitive to voltage spikes.