My humble opinion is to focus on the preamp. If you go with a tubed pre, the world is your oyster for rolling the sound and making it what you want. I don't believe an amp is the answer to change your sound if your amp is already properly paired with your speakers. Note that I'm not saying they don't make a difference, I just think it is less as apparent as a pre or DAC change.
As far as my experience with power cords and my SET amps, they are the one component I haven't heard any difference with when changing power cables. My system is likely not revealing enough at this point, but after trying a half dozen different cables, I've settled on the generic PC's supplied with the amps. I plan to revisit that area once I make a significant jump in my system again.