If you can find heavy gauge ( at least 12 with 10 being preferred ) twisted pair, that would be excellent. I know that Anixter used to stock 12 gauge solid core twisted pair. This design will lower inductance, especially if you've got a long run. It is bulkier and harder to pull through conduit, but it should offer some benefits.
While i don't know how "lawful" this is in various States, you might want to run this by your electrician. You can use the conduit as ground and isolate it at the breaker box. Install a dedicated ground rod for the audio system in very close proximity to the ground rod that is used the mains and ground the conduit to the secondary ( audio ) ground rod. The two ground rods should be tied together electrically and weatherproofed once you have very solid connections.
Since the rods are tied together at the point of Earth ground, noise from the rest of the house will be shunted at that point. This is especially true since you now have two ground rods, giving you twice the conductive area to ground. Since electricity typically prefers to take the shortest, lowest resistance path to ground, any type of noise coming from the house / AC mains would be shunted at the point of connection.
The system now has a dedicated ground AND is connected to the rest of the house in a manner that should be both legal ( building code & home-owners insurance ) and harmless in terms sharing a common ground. On top of that, your AC feedline to the audio system is basically a 100% shielded low inductance twinaxial cable. This saves you from having to run the third wire and leaves more room to pull the twisted pairs through. Sean
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