For Krell1: I am not trying to trash Krell, they obviously are an expensive and heavy amp and use good-sized transformers. BUT that is not nearly the whole story in power supplies. As I mentioned, Ayre is one of the few companies I am aware of that goes to the trouble and expense of a choke-input power supply (i.e., a giant series inductor nearly as expensive as the transformer itself). This makes a gigantic difference in the quality of the power delivered by the power supply, and Krell and most others don't bother. And my ears tell me that it makes a huge difference in sound quality, on the same order of magnitude as adding a really good power conditioner (as you would expect...).
There are many other factors that come into play as well, including capacitor quality and sizing, rectifier types, and so on. In addition, there are other potential solutions that can deliver excellent results, including the excellent new switch-mode supplies on the Linn Klimax and Rowland 10/12. A good source of info on this is Pass's website www.passlabs.com. I am not saying that any designer can make a great power supply easily or for free, but that it certainly can be done if they are willing to put out the effort, and most (including Krell) haven't bothered to take it to this level. That is why some products benefit so significantly from power conditioners, and others either don't seem to care or, if they have really good power supplies, can actually sound worse with conditioners.