Agreeing with Hamster and Big Cheese here really.
Mains cables cannot make a difference - we are talking 50Hz here. Cables have effectively 'no' loss at this sort of frequency, and ALL mains is dirty. Screening them makes not one iota of difference as the ring main in your house is a far better radiator (it's a ring after all!) than a metre of flex will ever be! External power filtering is a waste of money also. If it makes a difference, consider writing to the manufacturer of your equipment and suggesting they hire someone who understands power supply design, rather than someone who believes in fairies.
Taking money off hi fi folk is like shooting fish in a barrel. Not many *want* to believe in physics and engineering because it would reveal they've wasted a load of money on snake oil.
However, there is solid reasoning why cables sound different. It isn't down to the shite that eBay sellers spout. Like Big Cheese says, it's down to the electrical properties of the cable - impedance - which is a function of it's reactance, and how that matches the components innate impedance. Speaker cables affect things like damping factor. There is no magic, honest.
However, if you want to spend sh*t loads, drop me a mail, I can help with that
I'm not trying to be a smart arse. This is what I do for a living. I know a little about this sort of thing, I manage the Automotive department of a fairly large EMC test facility
"Two very small men cutting steps in the roof of the world"