fabia amplifier engine noise, completely baffling me :(

i have a 51 plat fabia comfort (hatchback) 

i have fitter an amp and speakers, 

i have made sure the ground is correct and even tried different grounding points

i have even tried a ground loop isolator but all the seems to do it make the volume about 80% quieter and take all bass away :( so I'm left with tin like music and perfect rear speakers link to the head unit.

my rica cables are routed on a different side of the car to the power cable and i have bought the most expensive rica cable marlin had so it is shielded and protected but still when i turn the ignition on all you get is a horrible noise that gets worse when you turn the engine on making it impossible to listen to music or my radio