This is a really interesting blog on power conditioners:
Like me, this guy has an AudioPrism Noise Sniffer to determine the line noise on any AC outlet. But he's figured out how to get repeatable measurements by measuring the audio output of the sniffer with a SPL meter. So he's got numbers that show that sinewave regenerators (like PS Audio Power Plant) produce the quietest power, followed by transformers. Noise filters in the more expensive outlet strips also filter pretty well.
I'm thinking that the noise filters in my Belkin PureAV AVU1500 are doing a pretty good job and that is why I got some of the depth back in my system. The quieter backgrounds help with depth perception.
I just did a simple noise test comparing industrial grade Smart UPS 1500 (a 70 pound unit) with the Belkin. Unfortunately it was not a good test, as these big heavy units were in different rooms plugged into different outlets. But while the Belkin had basically clean power, the Smart UPS power (with normal A/C input) was buzzing away badly.
I'm a little unclear how the Noise Sniffer works and how best to use it. It's my take that the volume control also changes the frequencies that the sniffer is testing, so you always have to run it through the whole range.
I have never used any power conditioning device with my Krell FPB 300 amplifier. It gets power from dedicated circuit with isolated ground, through audio quality outlet (currently Oyaide), and that's it. I'm afraid that anything might melt under the immense current draw of this amp which challenges the 20 amp circuit that it's on. And anything might restrict that power also, obscuring some of the Krell magic.
The Krell has been offline now for three weeks, but I'm using the same philosophy with the temporary substitute amp, a Parasound HCA-1500A. Which is actually one of the few analog pieces in the system, and it might actually benefit from some kind of conditioning.