I have NOS Mullard el84’s that I use in a Leben 300xs. They are special tubes. I would seek them out again along with some other NOS tubes. I
I’ve owned many amps over the years, even some of Kevin’s amps, and can say they make a difference to me. A sense or ease, a window into the notes and texture that is unmistakable to me. I do agree power tubes typically matter less, but if you want to go further you can.
If you have the funds to experiment with old and new I recommend it, but tubes are all magical in their own way so play within your comfort level.