Serial #26 responding... I've upgraded to a pair of Takatsuki TA-2A3 power tubes, an Elrog ER-5U4G rectifier tube, and a Sylvania Gold Brand 6BQ7A input tube. I'm not very skilled at describing the subtle sonic characteristics of these things. The Stellaris is my first tube amp so I'm not drawing on lots of past experience with different tube configurations. I will say two things: First, I find the sound with the upgrade tubes to be a bit more
lively relative to stock. I like the change. Second, the Stellaris sounds outstanding without the upgrade tubes and new owners (and budget-constrained owners) should not feel compelled to make the additional investment - especially not right away.