Andrewdoan, I tried that same power cord and it changed the sound in a negative way. System sounded very dull, undefined and with rolled off highs. I gave it a full 30 day break in and returned it. You are right, the stock cords were better!
Power cords do change the sonic signature...it's just a matter of which way you want to change it. PS Audio Prelude happened to change it in a way I didn't like.
There are lots of very good power cords though. Basically, for about the same money as PSA Prelude, Shunyata Diamondback was a big improvement over a stock cord in my system. You may want to try that on your amp. Open sounding with nice dynamics and has positive effect on soundstage and overall sonic picture. Similar to Diamondback in price and sound(smooth sounding with a bit more emphasis on mids than Diamondback), is the Acoustic Zen Tsunami Plus. Both cords should be way better than your stock cords.