New listening room electrical design

Maril: Some thoughts to consider on your narrower set of questions. If you have one line feeding multiple receptacles or receptacles plus a distribution box, you have the advantage, I think, of eliminating the different grounding potential among separate dedicated lines. I think this is the "British" style, if I can label it as such. Downside would seem to be that you are limited to what- a 20 amp line shared by multiple devices? (Leaving isolation of the digital to one side as you suggested).
You may remember, I also have the ML2 and you are right that they don’t draw a huge amount of power, but I know Vlad always harps about not using conditioning to bottleneck the current to his amps. I don’t know if a shared line (even of all analog) would be limiting.
I also have the Duos (though if I recall, you have newish ones). I wound up installing 4 dedicated lines in my room. The downside to how it was done was that all the lines were bundled together when the conduit was brought up from the basement, so when the air compressor motor for my tonearm kicks on, it affects the other lines. (This came up in another thread- I think Jea had pointed it out). So there are some best practices, not only to gauge, per CLeeds, but other aspects of how the cabling is done, from panel to room.
I’m not sure you can have a local distribution box in the room that is hardwired under applicable code, but Jea, or someone else here with real electrical knowledge and skills can address that better than me.

It’s great fun to plan a new room- i’ve been doing it mentally for several years, waiting to relocate. I guess the biggest limitation is that this stuff is "in wall" and once done, it’s a mess to re-do or add on to. Dunno if that helps....

I always learn something from Jea. Question: unless the construction of the drywall/studs is  already first rate, won't it be way easier to tear it out and start from scratch? 
Re Jea's comment about the proximity of the subpanel to the outlets, couldn't you locate the subpanel toward the front of the garage, to increase distance to the receptacles? (I guess that means a greater distance between the main panel/service entrance and the subpanel and I don't know what code implications there are for that).
FWIW, I used some of that mass loaded vinyl (comes in rolls and man is it heavy) in my adjacent mini-room as part of a sound proofing exercise, and I gather that it is intended to give drywall construction far more mass (apart from the soundproofing application I used it for- coupled with those acoustic blankets in my case). 
I'm enjoying your research phase vicariously, and taking notes for my own benefit. 

Maril- I was curious about the solution to the comment Jea had made up thread. There, Jea said:

"JMHO, if the subpanel will be somewhat close to the audio equipment’s branch circuit wall receptacles (15ft or less) imo, that kind of defeats the purpose of multiple dedicated branch circuits for the purpose of decoupling the power supplies of audio equipment from one another. Especially Digital from analog.

Imo, the amount decoupling is largely due the length of the branch circuit wiring impedance of the branch circuit wiring used, especially NM-B cable (Romex is a trade name of NM-B), or MC cable. (Both use THHN insulated copper conductors.)"
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I don't know the answer, in fact, I had never realized that was an issue, so that's what prompted my question. 

Maril- for my own info, what led you to the Soundbreak product? (I will eventually be doing a build out too, that's why I'm asking). 
Sounds like you are well on your way.
Exciting! 

Maril-Thanks re the material- yes, I suspected pulling out the old drywall would make everything easier and better. Glad you did. Let us know what you think of the sound once you get it up and running.
bill hart