Tube Rolling Leben CS-600X

The 6dn3 is to mitigate power on current so no benefit to rolling. You might consider NOS Tung Sol 5881 for the power tubes. Get a nice pair of 12au7’s, perhaps some old Mullards or Brimars, it all depends on your tonal preferences. Perhaps even some nice RCA Command 5814a’s for a little more neutral presentation. Get a nice pair of 12bh7’s, NOS GE’s aren’t terribly expensive. Those tubes should be matched and I’m not sure how you ended up with the odd pair in that position.

For the price of those blue glass el34’s they better be good. I also believe Jeff Day recommends tubes that are widely available rather than the best sounding. Just a guess. I think the best sounding tubes are the ones you roll and choose based on your listening preferences. There are many, myself included, who believe the NOS GEC Genelec KT66 is the power tube in that amp that makes you question whether investing another dime in audio gear makes any sense. It’s that synergistic and that good but they are pretty rare.

@kba_8040  - I don't have the Leben, but to ghasley's comments about different tubes and their sonic signatures I provided below the link to BrentJessee's website - a tube seller. You can look up the specific tube type e.g. 12au7 and aside from listing what tubes he has in stock he gives his listening impressions on the various brands as well as the respective variants for instance 12au7 - 5814 - ECC82 - 7316.  Perhaps a look will be helpful to you.

https://www.audiotubes.com/

@ghasley and @facten - many thanks for the comments.  I've heard a lot about EL34s and other tubes, nothing about 6dn3, understanding the role it plays in the amp explains why it's not part of the tube rolling discussion.  Good to know, too, about the KT66; the Leben has switches to change the cathode resistor and plate voltage setting to accommodate different tubes, along with a chart in the owners manual regarding the tubes and settings, I just need figure out what an X, O and triangle shape mean (see here:  https://www.atconsumer.es/wp-content/uploads/CS600X_Manual.pdf)

Appreciate the link to audiotubes, I do find the subjective opinions about tube sound helpful before buying.

A final new-to-tubes question:  for the sockets in the Leben (I assume other amps, too) labeled for example as "EL34", would any tube indicated as EL34 work?  For example, the Sophia EL34 tube is labeled as "EL34-ST" - is an EL34 the same thing as an EL34-ST or is an EL34-ST not interchangeable with an EL34?
 

ST is the tube shape. Non traditional names just do your research to ensure the electrical characteristics are similar enough. Many el34 amps must be populated with el34 tubes that adhere to el34 specs. Since the Leben accepts 6l6gc's you have more options.