If a power cord improves something to your ears, it's an improvement period. Who knows what the manufacture intended the component to sound like? Do you use the same components the manu uses to voice his equipment? Or -- using a stock power cord, would you know what the 'intended' sound of your preamp was while listening to it connected to your source, amp and speakers by your choice of interconnects and speaker cable?
It's your system -- you determine what sounds best, just different, or worse.
It's your system -- you determine what sounds best, just different, or worse.