Power conditioner or not?

As long as the Naim gear doesn't already have some sort of line filtration on board, then I would suggest the only power conditioner I know of that works so well, you will become addicted to it! I've tried most of what's available and was dissapointed. The MIT Z stabilizer MKIII HG is not only a bargain at $699 msrp, but it will transform your listening experience. Dynamics, both low level and macro are increased. Dimensionality and instrumental tone is beyond reproach. My solid state gear became so organic and expressive that it sounded like what I would expect from reference tube gear but without the drawbacks!!

I have noticed the same improvement with power conditioning with video. With audio the difference is very small or not noticable at all.

I sold high end audio (including Naim) for many years and know many music lovers with good audio systems. Of all the strategies for dealing with AC problems, the one universally reported to net positive results is to install a dedicated line for your equipment.

Naim's recommendation is most likely based on their listening tests, which they, I assume, found most PC's to be a negative. Since they cannot (or will not) endorse one or two units that they may have found beneficial or neutral, they'll take the easy way out and recommend none.

Another factor to consider is that some power supplies do not have oversized transformers and/or filter capacitors in order to get the sonic benefits of low PS output impedance. But these power supplies are more sensitive to current restrictions than beefier ones. Regardless of what the marketing wizards say, all PC's will impede instantaneous current to some degree, lest they found away around the laws of physics.

I concur with the recommendation of the MIT Z-Stabilizer III HG; a fantastic product that to my ears has no detrimental effect on the music and is a stone cold bargain at the pricetag (I have four).