What power cords are you "upgrading" from? And why did you decide to start at the amp and then next to the preamp? My experience with power cords is that the digital components and phono/line stages react far more to power cords than do amps.
The logical thing to do would be to try a single new cord in the system, at each of the 4 places and leave the other 3 original in the system. You do not need to live with this change for days or weeks. The change should be noticeable instantly. Following this experiment 4 times, you will have an idea as to what components in the system react the most to the cord change. Then you can decide what is the additive benefit of 2 or 3 or 4 of these new cords.
The logical thing to do would be to try a single new cord in the system, at each of the 4 places and leave the other 3 original in the system. You do not need to live with this change for days or weeks. The change should be noticeable instantly. Following this experiment 4 times, you will have an idea as to what components in the system react the most to the cord change. Then you can decide what is the additive benefit of 2 or 3 or 4 of these new cords.