I noticed the same thing. Like, why not at least use a fullwave rectifier with semiconductor diodes? Surely nobody believes that a tube diode in the power supply makes any audible difference.

Sometimes it does, for example, just like gp mentions, a tube rectifier in a single ended amp can have a voltage “sag” that interacts with the rest of the system and causes an interactive “color” in the output, especially when amplifying larger voltage swings of bass notes and chords.

There are quite a few effects like this. In a modern design this would be eliminated, but sometimes “bad” is good :)

What, like a 5Y3? Listen to Neil Young’s tone on Cinnamon Girl. That’s the sag and compression of a cranked tube rectifier.