Per post 1596
Ccomment 1. Yes, this is where I find metal encased inwall wire to be a benefit. It is very good at rejecting EMI from surrounding wiring. I know that is not the intent of your comment. But its the point that EMI radiation is a real issue. Mostly with EMI getting into signal or speaker cables. But I am cautious of it with branch circuit to branch circuit as well.
I also find any isolation transformer has to be located strategically to not interfere with the equipment it is powering. That is why I prefer a wallmount in the adjacent room.
Per comment 2. I do not find the sine wave distortion from audio equipment PS to be problematic if you run dedicated line. That is partly why I strongly recommend multiple circuits. I do not like a single branch with the assumption it is ridding ground loops. PS to PS noise can become an issue on a single branch carrying all the equipment.
My comment about the issues being in the equipment itself is exactly that. For Example, a 4 year old set of $60K monoblocks with buzz at the tweeter. Owner gets the amps updated and all the buzz is gone. A brand new $22K phono stage with radio coming out the speakers. I insert 2 other phono stages and the radio is gone. I did a lot of other test to validate the issue was the phono stage. $15K SET tube amps with bench tested measured voltage bleeding from the PS to ground creating ground issues. I see gear itself with issues. Not the power feeding it. People don't want to believe their expensive purchases have defects. But that is many times where "Issue" are traced too.
Updating your power is not about solving issues. Its all about extracting the maximum performance from what you have. If you have hum and buzz, its most likely the equipment, not the electrical supply. If you have a hazy veil, soft uncontrolled bass, fatiguing sibilance, it very well may be the electrical infrastructure.