dedicated circuit, flex conduit?

Flex conduit (BX) does not ususally come "prestuffed" with wire. In older applications the BX conduit itself was used as the circuit ground. You would definitely not want to do that in a modern circuit as it would usually not pass code and would be suseptable to RFI/EMI. Do not share a ground with two different circuits, you could introduce a ground plane hum and also have code problems.

My own personal soap box is the use of twist-lock recepticles and plugs rather than NEMA 5-15 or 5-20 "straight prong" connectors. Twist-lock offer surface contact area several magnitudes greater than straight plugs as well as self cleaning contacts and a mechanical lock. These outlets can be installed in a normal box. Give it a thought.

If you do not have a 200-300 amp main panel you might consider upgrading your main panel service at this time.

Personally, I would not worry too much about the concerns you have. Which is good for me since I could not answer them.

I would focus only on the following:

1. Obtain good aftermarket 10gauge 99.95% OFC romex-like house wiring.

2. Ensure that only the amplifier(s) have 20 amp circuits/lines and outlets. All others are 15amp.

3. Obtain aftermarket 20amp audio-grade wall outlets for each dedicated line.

4. Ensure that all is properly installed.

5. If you only intend to install two dedicated lines (why not three or 4?), then you might want to consider placing ONLY the amplifier on the dedicated 20amp line, the preamp on the dedicated 15amp line, and the digital on another non-dedicated circuit.

-IMO

Just to confuse you even more, these all are "flexible" type cables:

Armored clad types AC (e.g., BX) and MC;

Romex family NM, NMS, NMC;

Flexible metal family FMC, FMT, liquidtight FMC and liquidtight FNC.

All of the above come with insulated copper conductors. This wire is called THHN, which is the designation for the plastic jacket that covers the copper. All the nonmetallics have a grounding (green) conductor as well as the hot (black) and neutral (white). What you use depends on the application but AC is the most common, with or without the green. I would suggest the MC type, which has a green ground. Isolate the ground, meaning attach the green wire to the green screw of the outlet, which itself has a spacer to isolate it from the metal outlets mounting box.

I have successfully used the Belden 83802 no-conduit, double-shielded, all_teflon 12AWG fire-alarm cable for dedicated lines for a couple of years, and also make PCs from it. Email or call me for details. Cheers. Ern