So, back to the earth wire onto a RCA plug, are you sure that the RCA plug MUST be ground to the circuit????
99% sure. You can increase that to 100% by checking for zero ohm continuity between the ground sleeve of an RCA input jack and the ground sleeve of an RCA output jack, doing that while nothing else is connected to the preamp.
If so, could I chop off a rca plug from a unused rca cable I have and solder this up?
Yes. Make sure, of course, that you solder up to the wire that is connected to the RCA's ground sleeve, and not to the wire that is going to the center pin.
So I measured it, using a RCA plug outer and a chassis screw the result is 0. Does this mean I can use the earth wire to a chassis screw? .... tried it anyway. Yes, the cone of the driver moves, but move the gain up above 50% and it makes this horrible noise .....
Assuming the measurement was done properly, and the result was truly 0.0 ohms, I would expect that to work. Perhaps when the gain is above 50% you are overloading something with too large a signal, causing severe distortion. Try a higher frequency that is more readily audible, such as 50 or 60 Hz, with the earth wire connected to a chassis screw. Start out at a low gain setting and see what happens as you raise it.
The fact that you are now seeing cone movement is encouraging.
Could I just try and Hold the earth wire onto the outta of an RCA plug to try it and see if that works???
I wouldn't recommend doing that. If secure contact isn't maintained from the time the equipment is turned on until a minute or more after it is turned off (so that the power supplies in the components have had a chance to discharge), you could put a large and potentially destructive transient into the sub and/or the main speakers.
Regards,
-- Al