WOW, I see so many unsupported Comments here, Questions Unanswered, and pure NONSENSE. And I am not taking any side YET on the original topic. I have a cancered view of most professional power conditioners. To start with they have the ENTIRE feed going through a tiny trace on a circuit board. My own conditioner that I MADE, doesn't use such circuit traces but actual wire.
Next, I have great surge protection but most people don't even know if the MOVs in their order unit are still viable as they can be destroyed by a surge and you will never know it.
Then I use some commercial power conditioning modules that are as good as far as the schematic goes as most useful in other high-end models.
Finally, I DO utilize a couple of external pieces the 'Nordzt Qkoil' which should counter what one person commented here "If a power conditioner is squeezing (throttling) the power then it demands to be removed."
and
the ifi 'Ac ipruifier'!
My entire box is capable of 20 watts but of course, I only use a 15-amp circuit from the Wall. The only improvement I might like to have is a better quality of COPPER in my box, end to end, BUT it is no worse than the wiring in my walls so, I doubt that it causes much of a negative effect.
Good grounding Good quality sockets, having the type and number of sockets desirable for my usage is a priority, and you can never get that from a commercial box.
I realize that not every Audiophile has an Electronic Eng degree but you don't have to have one if you can look stuff up on the web and ask a few questions from people that DO KNOW. Then all you need is a little skill in constructing a box and being able to handle a screwdriver and a few other hand tools.