Agreeing with Hamster and Big Cheese here really.
Mains cables cannot make a difference. 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