Thanks for doing the research and posting the clips.
You might notice more differences without the attenuator, although 12dbs is not an extreme amount of attenuation, it might still make a difference.
One of the beauties of the Two Stroke design is how harmonically rich it is. Putting anything in the path to the speaker could have an influence.
I’ve noticed significant differences between tubes from different manufactures, different choices in power tubes (of course), different rectifier tubes and NOS vs. new manufacture tubes.