I don't care what some audio manufacturer told you to do. I am telling you in the USA, the NEC is absolutely clear you will not have a isolated ground system on your property. It has to be tied to the primary ground system your utility power is grounded too. No ifs ands or buts. I know the safty reason why, which you seem to feel don't apply to you or your family or neighbors. I am assuming its a fairly universal rule as the reasons behind it are centers around life safety.
I have told 2 customers of mine with Groundmasters I will not engage in a non code compliant installation. But I will happily bring a dedicated ground wire from the systems grounding electrode to the room. Both of those people said their ground master performed just fine for them connect that way. That is the electrical life safty code compliant way to conect a ground box/system to earth. Do not put a loose rod in the yard.
Even of you don't worry about killing someone, another reason is lightning. For some reasons when lightning strikes ground it has a tendency to walk up the ground in your house and out the the utility ground and back to earth again. It does weird things. Anyhow, a manufacturer of industrial machines was telling their customers to drive a ground rod into earth and to tie the equipment to it. Their machines kept failing. It turn out lightning was hitting ground and going up the extra ground rod, though their equipment and out the utility. I'm pretty sure a lightning strike to earth close to a home will blow right through any safety devices built into the groundmaster and fry your gear.
It works the other way too. If it hits the power line its racing to your panel and trying to exit out the ground to earth. If you have a ground rod out in the yard, some amount of that surge might race down the branch wire, through your gear and out the ground wire to the rod you stuck in the yard. A lot of factors at play, but its a very real possibility.
Enough said. Don't put a loose ground rod in your yard.
