An unconventional approach to POWER MANAGEMENT

If you have the space in your main panel for a dedicated circuit or circuits and don't have a very long or excessively complicated run, what is the logic for a sub panel?

As MC pointed out, it's just extra stuff.

With respect to following code, some of the setups I have read about on AG are irresponsible and downright scary! 20A outlets on 30A circuits! There are standards and ratings for a reason!

I have four panels, my very own high voltage transformer and a whole house generator. None of this was aimed at my system. Assuming your house was wired correctly to code, putting in a separate panel for your HiFi system will do absolutely nothing but waste money and any good electrician will look at you sideways and say something like, "are you out of your mind?" I can turn a 10 horsepower three phases shaper on and off and it has zero effect on my stereo. 
Good equipment does not need any of this power management stuff, none of it. Spend your money on good equipment not fixes for bad equipment. But to tell the truth I have never heard bad equipment need it either. 

If you have the space in your main panel for a dedicated circuit or circuits and don't have a very long or excessively complicated run, what is the logic for a sub panel?

As MC pointed out, it's just extra stuff

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True. If you have space on the main panel, wire the dedicated line on one leg and move the high-current devices and appliances to the other leg. It's been recommended to install these appliances as far as possible from the audio line on the panel.
I know, I know, these circuits are shared in the panel and not isolated, but there may be a benefit of less noise from appliances like a/c or furnace switching on and off. 

Everyone has space to add a dedicated line. Even if it looks like every slot is taken, you get what is called a thin twin, or maybe it goes by different names with different brands. This is a breaker designed half as thick to allow 2 circuits where normally one would go. 

So probably everyone suspects the thin twin is not as good. Okay fine. So get your thin twin, move one less critical circuit to that, use the empty space for your dedicated line. 

Either know what to look for or get an electrician to examine your panel box they don't all allow tandem breakers.