Power cable question/observation

NotaSuv

I have had fun upgrading PC's and have heard a improvement overall with each upgrade nothing real fancy..now I have been having a awful ground hum which I thought was my cable box for the longest time....turns out that two of my amps seem to like a very cheap PC with a cheater on the end...the Dayton 1000 sub amp and the Proceed hpa2...with anything but the cheapest cord there is a noticable hum...any reasons why a better?? cord would produce the hum? This is the first time I have run into the cheap plain one is best..have swapped around lots of cords tried different outlets and in the end the cheapest cord wins BUT only with these 2 amps. All my other equip benefits greatly from a PC upgrade..