Upgrading power cables

Thanks for so many informed responses!

I agree with upgrading the source first. But, don't discount the importance of a good PC on the preamp. The preamp influences the timbre of your system, especially a tube preamp since tube rolling can change detail, soundstage, dynamics and colour.
 
Finding the right PC for your preamp can influence the sonics dramatically. One thing to keep in mind is that the preamp is a low-current draw component and many aftermarket PC's have heavy copper windings more suited for an amp, so

Since you are thinking of upgrading the power stage, I'll second the recommendation of adding a dedicated AC line (if possible). This line will provide you with cleaner, more stable current, since you will now be isolated from the rest of the devices in your home.

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.

The power cord is the most important cord in your whole system.  Change all of them.  Its easy to make your own.....use top grade components. (oops...I found no difference in the power cord to my turntable...but other than that one....)