Lack of noise. Fast power delivery. Lowest noise will allow detail, nuance, (dynamics- as lowest sounds heard) :)
Why particularly the preamp, have you solved the power amp cable?
Just changing the preamp's power cable alone will not bring the upgrades you are looking for.
My suggestion is borrowing a set, for both power and pre, from a dealer and listen how it goes, if you like the change decide if you want to spend more or less.