Which power cord to upgrade first?

Tvad, your suggestion comes right on time. I need a cheap cord now while I am waiting for my Audience cords to arrive. I have all the stock cords burried somewhere in the closet and I'm too lazy to dig in there for them.
I may consider the Volex.

Basically any component should benefit from better power cord. The question is - Is the rest of your system good enough to realize the change?
Upgrade power cords(although not all of them) do improve the sound, but the improvement is much smaller than what you would get from IC or speaker cable upgrade.

I think in your case(with a receiver) it's the source that should benefit from a better power cord.
One power cord that I found to work equally good and make a noticable improvement for both digital and analog components is Audience powerChord. It is unshielded, but an excellent cord for either source or the amp. Good luck.

Tvad, $5 cord? that's cool, but I said "basically". And the better power cord in your case was the $5 cord for that particular component.
It's a combination of things. Not necessarily one upgraded cord is going to make a huge improvement. Have you thought about what the rest of the power cords in your system are doing? Is it possible that all the other cords there in your system did more harm than good in the combination with your pre-amp and then you drop this el-cheapo cord that in the context of another system would probably sound like crap. And all of a sudden things come to life?
I appreciate what you're saying, but the most important thing is system synergy. You achieved it with the addition of that $5 cord. Congrats. :)