How many power supply devices do I need?

There is no "isolating". The thing about my humor, once you experience and understand what's going on its the dude's post that's really funny. Seriously. Just plug em in.

ooooohhhhhhhhh.

Why do you need an external LPS on the amp and AVR?  

The HDPLEX looks interesting with it's variable voltage. But it does not indicate amperes.

I owned a Chord and it really does benefit using a LPS.

I dunno, it seems everything needs a power supply.

thanks for info on chord dac.  

Amps and AVR’s have internal power supplies that are sufficient to drive the component. They could be switch mode (SMPS) or some type of linear. On the back of these components most likely there’s a IEC connection, which you plug your power cable into.

The Chord Dac has a small internal SMPS power supply and is able to accept a larger linear PS thru the plug in the back. Typically a linear PS is less noisy and more robust which can improve SQ.

The Roon also accepts a LPS, but I've never used one. You need to ask other Roon owners.