My system is powered from a P5. I recently picked up a Power Base, with the intention of putting it under the P5. Before doing so, I decided to try it under some other components (once in place in the bottom of my rack, it will probably never come out).
Right now the PB is under my preamp, serving only to provide vibration control. (Noticeable improvement, BTW.) I don’t think there would be any advantage in plugging the preamp into the PB, fed from the P5, as opposed to connecting the pre to the P5 directly. The power coming into the pre is already clean, and another cord between the PB and the pre would add its own sonic signature (probably not for the better).
The exception might be a component that is some distance from the P5, in which case the long power cord could pick up noise that would be filtered out by the PB.
Does this sound about right?