@iFi audio thanks for your response.
In the meantime, I have done some serious experimenting.
RE: "not adequate" earth and both polarity and Earth LED red:
I've measured the voltage on my wall plugs. I have 229V phase to neutral, 80V earth to neutral and 140V earth to phase. In other words, my earth has 80V on it. I assume that's why both LEDs on the AC iPurifier stay permanently red.
I'm only mildly surprised. Here in Thailand, the earth is generally considered a useless extra wire that is best cut off to keep things tidy... I half expect that my building earth is buried in a concrete wall somewhere. I guess my only option is to make my own earth. Dedicated line for audio... oh dear, where am I heading?
RE: placement of the AC iPurifier, "before" and "after" considerations:
I have tried all kinds of different positions. The overall conclusion is, the higher up the chain the AC iPurifier goes, the more difference it makes. It took me a while to notice the difference, but now that I know what to listen for, it's obvious. With the AC iPurifier in, the mids become less shouty and more relaxed, the bass tightens up and is more impactful and there is more detail in the treble. It's the same kind of improvement as iPower 5V for my USB but much subtler.
However, the treble had this weird metallic, fatiguing quality to it. I removed the AC iPurifier and ended up prefering the sound without it, despite the slight loss of detail.
That got me thinking - this electrical noise affair is no delusion. The higher up I move the iPurifier, the more I filter, the more difference it makes. So, I thought, instead of filtering noise, let's try removing it.
I already knew removing my printer from my computer improved the sound. I had assumed that was because its power cable went right passed the USB audio connector.
I looked around on my desk seaching for anything else that used 220V AC. It was staring right back at me: the screen.
I unplugged the screen's power cable...
I instantly got this widening of the sound, tighter bass, more detail, but this time without the metallic harshness in the highs.
Added bonus: with the screen off, I'm forced to focus on the music. No messing around surfing the net. I've added some keyboard shortcuts in foobar so I can still skip tracks.
Conclusion:
Step 1 - I interpret all this as the AC iPurifier is doing its best and is removing some noise and also a small amount of signal. The sound therefore tends towards being less noisy. However, this is not a match for actually removing the noise in the first place. So I'm not going to be using the AC iPurifier in the foreseeable future but I'm still glad I got it for the learning process it has been. And, while not exactly dirt cheap, the experiment didn't break the bank.
Step 2 - I'm starting to understand the utility of these Music Servers and Music Streamers. They are turn-key solutions to this electrical noise reduction process that I'm going through. But it would kill all the fun.
Step 3 - I'll try unplugging the LAN cable next. That also goes to a router with a SMPS. After that, I'm going to have to ask everyone is the house to switch off all their devices whenever I listen to music, including the Wifi, and possibly wrap myself in tin foil...