RF is a surface animal, by this I mean it's only on the outside of the ground conductor. Copper braid or tinned copper braid (better) will work the best for a RF ground. A solid #6 wire is not bonding (RF bonding) your amp/coax/radio to ground. As others have said, DC ground is different from RF ground.
Also the type of torroid material you use will have a big impact on the ability to attenuate the RF on the cables, some are for low frequencys and some are for high frequencys. Not much over a couple hundred MHz...
I have seen shelded MIDI cables but they are $$$$$$. Good luck Mike KB5UKT