Typically tubes run at very low plate voltages do not instill tube like warmth or crunch as they are advertising. Surplus miniature tubes such as the one shown have been available for a long time. I have not heard people bragging about audio applications for these tubes. Before jumping in I suggest you look for user reviews that you can trust. IMO this tube pre-amp in your guitar is probably not the path to improve your tone.
I'm not saying that you should give up the investigation but the offer just looks like a kit product made up to use the surplus tubes they found. They don't even disclose the part number of the tube.