"Sockpuppet Yustabe" said:

Well, the 6CA7 is generally equivalent to the EL34
while the KT66 is generally equivalent to the 6L6

While that's undoubtedly true, the sweetness of a pair of KT66s
(especially in triode!) can't be beaten by any other tube.
Just my humble opinion, of course.
You're right in that they're not directly interchangeable.

--
Sander deWaal
Vacuum Audio Consultancy