I thought that you might enjoy this article. Note that it addresses tube amps that are actually driven without load, not just sitting there like yours was. So the article looks at worst case issues:
BTW I love your amp and really wanted one, but it was just way beyond my budget. A less daunting situation was that it did not have a phono stage.
So, being good at working on amps, I purchased a Stromberg-Carlson ASR-433, which is a 12 watt EL84 amp from the golden age, with phono. I updated and upgraded it to assure reliability and proper performance.
Which lead to getting my hands on an Eico HF-81, yet another EL84 amp from the golden age........which lead to a Scott 299C, 7591 tubes and higher output.
And now I am working on a power amp, Acrosound Stereo 20-20 that too uses the EL84 tube. I love them all......but it was the little Leben that was my inspiration. Inspiration that has lead me down the path to perdition.....