Power cord for low-watt tube amp?

I imagine that the main function of many cords is to reject EMI and RFI as well as provide clean unimpeded power that a power cord for a smaller amp is still important. That is of course if you are a cable "believer" or not? In your case, intuitively speaking, the gauge of the wire, probably does not need to be very large relative to a 20 Amp cord intended for a 500+ watt class A amp.
Then againg I doubt that you are going to find yourself seriously considering a garden hose or Anaconda for your small amp. It seems that a very high quality silver wire perfectly constructed as in a Japanese type masterpiece is probably what your shooting for.
You might try Audiio Asylum cable asylum and search for PCs for small amp. That might give a sense of whether there is any buzz about it or not.

I would suggest an affordable option like the Pangea from www.audioadvisor.com or Signal Cable from www.signalcable.com IF you feel the need to upgrade. I use all Signal Cable in my system.

The latest thinking is to keep has much cash in your pocket and not pay for audio illusions that amount to delusions...

Yes, by those whose systems are not resolving enough to show the audible differences in cable.....but not for the many of us A'gon members here where such power cables can make a very significant difference.

I have found the higher gauge cords designed for power amps and conditioners are also a better match for tube line and phono stages as well as tube DACs that have tube power supplies. So my suspicion is that the same would apply to your amps even though they have low power-output ratings.

Not many companies actually make power cables for both high and low power consumption products. But one which does, Dream State, I have much experience with. When I tried each type of cable in my system, the lower-rated cables worked best for components like the CD player and electrostatic speakers, and the higher-power rated cables worked best everywhere else. The only way to know for sure is to try both types of cables on your amps. Good luck on this.

by those whose systems are not resolving enough to show the audible differences in cable

Have you determined that these people even have systems? I don't see any evidence that they do.