Most (but not all) EL34 amps can use variants like KT77. Sounds different and whether you prefer it to the EL34 is up to you. I prefer KT77 in my EL34 amps because it's a little bit punchier but YMMV.
NOS EL34 becoming extinct
There is a thread running regarding whether tube gear is worth buying if one cannot find NOS tubes. I have been watching the prices of NOS EL34 soar and supplies diminish ( finding quads is quite challenging ). As my budget has shrunk as I retired I am thinking the unthinkable ; buying new production EL34. Tyranny for sure! WIth that being said I wonder has anyone found new production EL34 worth a try? Before diving down the NOS rabbit hole I had tried a quad of Groove Tube and was quite underwhelmed. As always your opinions are requested but please no snark.