I assume when you grounded the amp's RCA that it was with the signal lead (along with it's ground) still connected? If so then I would imagine the earth loop becomes more complex at that stage. For curiosities sake, you could slide all the AMP RCAs back until they don't ground and try again and see what happens.

BTW, did you get any mileage grounding an AC RCA at the empeg end like I previously suggested?