You know you'd think big fat power cords would make the most difference on big power amps, but "conventional wisdom" seems to be that digital sources in particular benefit most. My experience, which is far from exhaustive, seems to bear this out.
Most hi-end power cord makers seem to want you to plug them straight into the wall, by-passing power conditioners. However, I feel I need the surge suppression, so I use a Monster PB1100, about as cheap a power conditioner as I think one should mess with to avoid doing more harm than good. I used to own a Monster HTS5000 and the lowly PB1100 is nearly as good and cheap peace of mind, probably $150 on the net. All my positive results with power cables were with them plugged into one or the other Monster power conditioners.
Mark