Power cords makes difference-Why?

I just discovered to my amazment that even power cords swap with the same high-end brand of equipment makes a difference. My story follows , I would like someone to shed more light on the phenomenon.( I know there exist power cords costing thousands, but never bought in to it.)Recently I added a surround sound preamp (Classe SSP-50) and multi channel amp(classe CAV-150)and center and surround speakers(Dunlavy SC-1 and SC-II'S). I have two channel system consisting of Classe CDP.5, Preamp CP-45,Amp CA-400 and Dunlavy SC V's as main speakers.The power cord on the SSP-50 has a little larger dia then the preamp CP-45. Last few weeks I thought I was using 'better' power cord with the CP-45. Everything else remained same for two channel(SSP-50 isolated) and the sound stage changed for worse.I was scraching my head frustrated thinking if any thing else I did could change my Audio nirvana. Then I recalled the powercord swap and quickly changed back to thier respective cords and wallah: My original sound was back.So my question: What constitutes a great power cord? Thanks for your advice in advance.