If you have 1/2 an ohm of DC resistance across your ground return you have big problems... 8meter of 10awg should measure less then .03 ohms...

Just like loudtdi says you'd need to measure resistance under load for an accurate picture of whats is going on anyway..... measuring voltage across the wire and current draw through it with your amps runnin sine wave **** out should tell you what you want to know....

Anyway, decreasing the resistance in the amps ground return may help but at a point you may also hit a point of deminishing returns.. you've still got two different ground points that are seperated by a resistance sitting at differerent AC potentials, which are also connected through your RCAs ground... not really much you can do about it except to 'break the loop' although as posted grounding all components at the same point may help