Ok, here are a few items that caught my attention:
– Stranded wire is most useful for cables that will be flexed regularly or are difficult to bend due to large wire gauge (it starts getting difficult to work with at 12 or 10 AWG). But the conductivity will about the same as solid wire with the same gauge. It can exhibit lower losses at RF freqs due to skin effect, but that isn’t really an issue at power line freqs. Speaking of wire gauge, AWG 14 is good for up to 15 amps, AWG 12 is good uo to 20 amps. Unless you are running power-hungry big vintage tape machines, AWG 14 or 12 should be sufficient for a studio.
– The twisted pair thing will indeed reduce loop area, lessening the susceptabilty of the cable to picking up magnetically coupled interference. However, if the cable is insided a grounded metal conduit, it is already shielded. It win’t hurt anything to use twisted pair wire, but is (IMO) a bit of overkill and perhaps unjustified expense.
– Adding a local breaker box won’t help or hurt. A breaker is basically a switch. The load circuit (your studio gear) won’t care whether the switch is in the same room or out in your garage.
But 60 amp service.. that requires three 12 gauge cables fed from three separate 20 amp breakers from the source (the remote breaker box). Or possibly a single AWG #4 romex fed by a single 60 amp breaker in the remote breaker box. 4 gauge wire is extremely difficult to work with, especially if you want to stuff it through metal conduit; each wire has copper of around 0.2″ diameter, not including insulation. (Note: this is one case where stranded wire would be far easier to work with than solid.)
The local breaker box could, un theory, have its own earth ground, but that violates electrical building code. The only legal way to ground the local breaker box is through ground wire(s) going back the to remote breaker.
Unless you plan to do arc-welding in your studio, 60 amp serice seems like overkill.
As far as running audio cables next to power cables: Best practice is to separate them. But balanced and shielded audio cables are pretty immune to nearby power cables.
Magnetic emissions from power cables are limited by loop area of the 2 (very close) AC hot and neutral lines. and amount of AC current. But capacitive coupling from an AC line into a nearby low level audio cable is a possible issue, especially for non-balanced audio cables. The relatively high voltage of the AC power can couple noise (hum) into the shield/ground of a nearby unbalanced audio cable. The capacitive coupling effect is an inverse square reciprocal; doubling the spacing cuts the coupling by 4x.
No criticisms intended here… It looks like you have done extensive research. If you haven’t already, make sure you also look into the local electrical building codes that apply to your area to set up an effective and safe aystem.