Conditioners are a bit like unnecessary heart surgery. You just don’t do it. That doesn’t mean they are bad per-se but you need a reason use them.
They all do different things, be it filter noise, stabilise voltage, frequency (exotic), DC offset, earth shunts to reduce feedback, and the list goes on.
Naim’s advice should be interpreted as not to use them without a reason. If both of the following conditions are true:
- You have an audible problem like a humming transformer or unwanted distortion from the speakers.
- You have accurately identified the mains supply as the culprit and specifically measured and identified what is wrong with the mains.
… then opting for a device specific to that problem makes total sense. The cure, in that case, is better than the ailment. Ideally, if you move or fix the mains, you then stop using such a device.
I think the only truly benign device is a balanced isolating transformer rated at equal or greater than the circuit, That gives galvanic isolation and blocks DC offset which seems to be a common problem in some rural areas. Such things are best installed by an electrician and put in-line on a circuit dedicated to the hifi though.