Yet another Grounding Question-Separate 'Earth'

ps- house has 200 amp service, all upgraded about 5-6 years ago when the house was massively upgraded by the prior owner.

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."<<
>>>>>>>>>>

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.

OK, not to add to the confusion, but to answer the questions:

We have 200 amp service, goes into a main panel with one big breaker, littered with all kinds of smaller 15 and 20 amp breakers (box is quite full), and an existing subpanel that appears to be set up for some of the kitchen appliances.
The new subpanel is a 100 amp box, set up right now with 3 Square D 20 amp breakers. I'll check to see if he installed a 60 amp breaker in the main panel to support this new subpanel.

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- only because of habit, and because i know Bill Hubbell, and figure his trust fund needs the money (He is a serious car junkie).

The run from the panels up to the room is easily 75-100 ft., depending on routing.
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. (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.

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).
Obviously, I will ask him when I ring him tomorrow. He won't be back to my house until mid-week, so I'm sure I can get the skinny from him, and let you know as we proceed. Thanks to all so far, and particularly to JEA 48 for all your follow up on this. Regards, Bill Hart

>>"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."<<
>>>>>>

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."<<
>>>>>>>>>>

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"<<
>>>>>>>>>

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)."<<
>>>>>>>>>>>

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