I worked for Roger Modjeski for several years. He was always of the
opinion that if you designed the power supply of a component properly
then additional power conditioning would not be needed. In his system he
used an inexpensive power strip, mainly just to have the ability to
power on/off all his components simultaneously.
@clio09 I've found this to be true too- you really have to regulate!
If regulation isn't an option then a power conditioner can do some good- but it had better actually be a real power conditioner and not a glorified power strip (which describes most of the 'high end audio power conditioners'). If you don't push them too hard the PSAudio power regeneraters are quite decent.
The best conditioner I've seen was made by Elgar for commercial industrial use. It has the ability to regulate line voltage and put out a clean since wave (with guaranteed distortion less than 0.5%) right up to full current, some of which can support 28 amps continuous! These units are older as Elgar got out of the business decades ago and so have to be refurbished with new filter caps for their power supplies and the like. But they do offer improved sonics!
Its the 5th harmonic of the AC line frequency that is arguably the most pesky. This can occur when the line transformers on the telephone poles are pushed past 50% of their total capacity of for that matter any transformer in the chain between the power plant and the wall outlet. The 5th harmonic is hard to filter out because of its low frequency (300Hz) so the smaller 'conditioners' simply can't do it. It can cause power transformers to run warm and make noise, diodes to radiate more 'diode noise' and synchronous motors to perhaps run the wrong way. You can google this topic; Fluke instruments has a number of articles available.
Most of the 'high end audio power conditioners' I've seen are deleterious to the performance of our amps; I tell our customers to stay away from them or only use them for the front end of the system.
Many audiophiles understand that power cords can have a sonic effect (which is caused by the voltage drop across them- they are subject to Ohm's Law like everything else in electricity); consider that a glorified power strip forces you to use one power cord for a variety of stuff which might include two monoblock amps. If the amps are rated to make full power with 120 Volts available, there's good chance this won't happen if the voltage drop on the 'power conditioner' power cord is significant (2-3 volts is not unheard of). If the power cord or the connectors it uses (including the AC outlet) tend to get warm that's a Bad Sign. I've seen this phenomena rob an amplifier of a good 40 watts of output power- under those circumstances its easy to see how this could also be audible :)
