Upgrading power cables

For me it was the CD player, somewhat surprisingly.  Long time believer in interconnects and speaker cables, i never gave much credence to the idea that power cables could make a difference.  Last year i started experimenting.  First, i replace my stock cords to the monoblocks with Nordost Shiva cables--more and better bass.  Then i replaced the Shivas with Aciustic Zen Tsunamis and used the Shivas for my CD and preamp--still more bass, slightly better soundstaging.  Very happy with my system, i thought i was done.  I got a deal on a used Nordost Vishnu, just one step up from Shiva, so i did a little A-B test.  With the preamp, no real impact.  With the CD player--WOW.  Slightly more bass but better soundstaging and air and space around individual instruments--really noticeable, for the first time.  Could be system/component dependent, but if i were to start over, i'd start with the source first, then the power amps.

For me, upgrading the amp PC first made a big improvement in the system as a whole.  Using a high currant cord I got much stronger bass, a wider soundstage and more detail across the board. 

Upgrading the source cords further refined the sound to varying degrees, but found good pairings after a bit of trial and error (I have a mix of Shunyata, Kimber Palladian and Clarus Crimson).

I just changed my CD player and tried the stock cord first (it is a pretty nice one as stock cables go), but an aftermarket cord was noticeably better, particularly in the bass, mids and vocals.

You usually match a power cord to particulat component so they don't have to or should not be the same or even the same brand. That said, if you want top of the line they might turn out to be the same, maybe.
It varies which component will be more sensitive to the upgrade, it is hard to advise. In your case I would probably first upgrade your tube preamp cord, then the source and the amp the last.

"Honey, looks like I'm going to need a power conditioner."

"OK, dear, whatever you need."