Yet another Grounding Question-Separate 'Earth'

>>"But, and here's the really critical question- my electrician has proposed a pair of separate ground rods about 10' from the main ground for the rest of the house electrical system, and in his view, the audio system subpanel would be grounded just to these new ground rods, not connected, by ground or anything else, to the rest of the house."<<
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Find another Electrician! You may want to ask what else he is doing that does not meet NEC and your local codes.

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Gs5556's post is right from the text book, NEC. Follow his post and you will meet NEC code.
The earth shall not be considered as an effective ground-fault current path back to the source.

Whart, glad to hear you will not be using a separate equipment grounding conductor and ground rods for the sub panel that does not connect back to the main electrical service panel grounding electrode system.
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>>"My electrician has installed a separate subpanel for the audio system which is 'upstream' even of the main breaker panel in the house."<<
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Just curious how did the electrician do this? Did he tap into the Hot conductor/s ahead of the main breaker, of the main service electrical panel? What size is the main 2 pole breaker, 100 amp, 200 amp? Did he install a new breaker and enclosure at this tapped location to protect the feeder that runs to the new sub panel? How did he connect to the neutral conductor ahead of the main service panel? How far is the sub panel from the main electrical panel? What size of wire did he install to the new sub panel? How many conductors, wires?

Whart,
>>"house has 200 amp service."<<
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I assume the 200 amp main breaker shares the same panel enclosure as the branch circuit breakers. Is that correct?
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>>"fed by a metal insulated cable that appears to be close to an inch in diameter."<<
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From your description the wiring assembly is MC cable. 1" in diameter, probably 4 #6awg THHN conductors. Maybe 5 conductors if the MC is Hospital Grade. I am just assuming the sub panel will be a 120/240 volt panel. 2 Hot conductors, 1 neutral conductor and 1 equipment grounding conductor. Wire ampacity rating of 60 amps.
Is that correct?
At the main electrical panel, the electrician will install a 2pole 60 amp breaker for overcurrent/short circuit protection for the new sub panel feeder and sub panel bus bars. The 2 conductors of the MC cable, usually black and red, intended for the 2 Hot conductors will connect to the 2pole 60A breaker. The white (neutral) and green (equipment grounding conductor) will both connect to the neutral/ground bar. At the sub panel the neutral conductor will connect to the isolated neutral bar and the equipment grounding conductor to a separate ground bar bonded to the panel enclosure.

What type of branch circuit wiring will the electrician be installing? What brand of receptacles are you going to use?
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EMI noise.
Just curious why are you not just installing another isolation transformer with electrostatic shielding and a new panel to feed your audio equipment branch circuits. Usually a sub panel is installed because of a lack of spare breaker spaces in the main electrical panel or the audio branch circuits conductor are very long and ground loop problems exist.

>>"It was my intention to have him set up a number of 20 amp dedicated lines from this new subpanel, strictly for the audio-only system, and rather than have him ground the receptacles at each wall box, to run separate ground wires back to the subpanel for each- not daisy chaining the receptacles to each other. I have ordinarily used Hubbell Hospital Grade receptacles"<<

New audio branch circuits should be dedicated circuits. Each with its own Hot, neutral, and equipment grounding conductors.

Nothing wrong with Hubbell Hosp grade receptacles. Just make sure they are the HBL8200H (15A) or HBL8300H (20A) type. Note the "H", that means non nickel plated. Hey do me a favor. When you speak to Mr. Hubbell will you confirm the gutts inside the 15A and 20A recepts are identical. And only the face plates are different to be in compliance with UL and NEC? I contend the gutts are the same, 20 amp rating.
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>>"The run from the panels up to the room is easily 75-100 ft., depending on routing."<<
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That would explain the new feeder and sub panel. The smallest wire I would of used is #3awg copper. If the electrician is indeed using #6awg, jmho, it ain't big enough, not for the distance for audio gear.
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>>"The only reason I hadn't considered another transformer for this system is that the one currently used in the video system is located in the room, inside one of the Mid-Atlantic racks, and I can hear it hum at idle, when all the video equipment is shut off."<<
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What manufacture is it? Is there possibly a midway point, or closer, to the audio equipment where the xfmr could be installed?
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>>" (When the video system is on, the fan noise masks that). I suppose one could set up an isolation transformer down in the basement, near the panels, but would that defeat the purpose if the outputs of the transformer then have to make that long run? Apologies in advance for my ignorance"<<
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Not ignorance at all. Your logic makes sense. If it had to be in the basement, by the main electrical panel, imo it would just need to be a little larger, more KVA. Kind of like a bigger battery.
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>>"OK, went back down and checked- no new 60 amp breaker that I can see in the main panel, to support the new subpanel. Is it possible that the subpanel is wired as if an 'extension' of the main panel, and that he fed it, and its array of 20amp breakers from the main panel, but before any of the other breakers (other than say, the monster one at the top of the panel?) This would explain his statement about setting it up 'upstream' of the existing breakers (sic)."<<
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Maybe he has not installed it yet. Or maybe you missed it. The breaker and wire could be larger than #6 with a 60A breaker. Just make sure one is installed after the the 200A main.
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Email this Audiogon member, Cincy_bob , he installed a 7.5 KVA Topaz xfmr with electrostatic shielding a while back . Ask him if his is noisey. I am sure he can give you some pointers.
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Jim

Whart,
How about an update. What did you end up with?
Jim

Whart,
You called your new dedicated branch circuits 1, 2 & 3. Is your new sub panel a 120/240 volt panel? Or is it a straight 120 volt panel? Does your circuit numbers match the factory breaker numbers on the panel front cover? If the sub panel is 120/240V are all the dedicated circuits on the same line, L1 or L2, in the sub panel but not one on one Line and the other two on the other Line? Audio equipment that is connected together by wire signal carrying ics should be fed from the same line. If not, this can be a source of noise.

For another source of noise, have you ever checked the AC plug orientation for the proper AC polarity of the primary side of the power transformers of the Amps and preamp? You will need a decent multimeter and a pair of tin snips. The snips, you will need to trim the wide blade on one of your ground cheaters. It will need to be trimmed enough so it will plug in either way into the electrical receptacle, outlet. If the ground cheater is the pigtail equipment grounding conductor type tape off the bare lug on the end. If it is the type with the metal equipment grounding strap either cut it off or tape it so it cannot touch the receptacle cover plate mounting screw. The screw is grounded.

Start with one of the Amps. The Amp must be totally isolated from the grounding system, of your electrical system. Disconnect the ics from it and the preamp. Plug the amp into the modified cheater plug and plug the cheater into the wall outlet. Turn on the amp. Set your multimeter to AC volts. Insert one of the test lead probes into the wall outlet ground hole, make sure you make good contact with the ground contact. Touch the other test lead probe to the back chassis of the amp. If the back is painted remove one of the screws and touch the probe to the bare threaded area of the chassis. You should now show an AC voltage on the meter. Write down this voltage measurement. Turn off the amp. Unplug the ground cheater and rotate it 180 degrees and plug the cheater back into the outlet. Wait a couple of minutes to let the caps in the amp to bleed off. Now turn the amp on. Connect the multimeter test probes in the same manner as before. Note the voltage reading. The **lowest reading** of the two readings is the correct AC polarity, phase relationship, for the primary winding of the power transformer of the Amp. If the plug orientation is correct the lowest AC voltage reading measured, the wider polarized blade of the plug for the Amp will match that of the neutral contact of the receptacle, (20 amp recept T slot).

Repeat the same test procedure for the other amp/s and the preamp.

http://www.boundforsound.com/tweak.htm