Extention cords are a good source of unwanted noise, especially hummmmm. Generally, the fewer power cables laying around, the better off you will be. Power cables and extention cords are rarely shielded, if ever, and put off an electro magnetic field (EMF) which is often picked up by other cables (mic/guitar) or by gear or instruments. Never use those "2 strand" extention cords, they are ok for a lamp but not for amps or recording gear. If you have to use an extention cord, a surge protector/power strip is your best bet.