Since I've had some more time off lately due to the pandemic.. I decided to install a couple sets of power tubes, bias them..play a bit..check, then leave them idling for several hours with routine checks to see what happens. I have installed two types of tubes, both EL-34s. One set was a Tungsol quad, the other a JJ E34L set. I noted that the JJs were far more unstable then the T/sols initially, but eventually settled down. I also noted that the next day after they had completely cooled the bias changed in both sets by as much as 5mV/A! To me it appears that the 'burn in' option that some tube vendors provide may indeed be a viable expense. What's your experience?