The EL34, the CLASSIC British tone, loved by many. The 6CA7 is the US designation for the EL34 and is complete interchangeable. The 6CA7 is technically a different tube and has a bit more of an American tone as well as being much fatter.
The E34L is a special low grid voltage version and is not always interchangeable.
The KT77 is an interesting tube and is a drop-in substitute for the EL34. This tube is somewhere between an EL34 and 6L6GC in terms of design and tone. It really is the best of both worlds.