power cord replacement...source or amp first?

is there a general rule of thumb as to where an aftermarket will make the biggest sonic difference? i have one (ps audio) that i have been using with my simaudio i-5 integrated, but i just moved it to my arcam cd player. i have another one (signal magicpower) on order, but wonder if there's a general rule of thumb as to priority when replacing power cords. source or amp first? thanks.