The 6LF6 was used by Futterman in some of his OTLs. They have high-current low-voltage capability. They have become rather expensive as they are no longer made.
These days the EL509 is the tube that replaces the 6LF6 although not directly. It is a powerful sweep tube and is also used in OTLs. There is a non-plate cap variant that could be used, but FWIW the KT90 was developed from this tube, so if you really want something like that the KT90 is probably the place to look, as the design of the amp is made simpler by having no plate caps and the use of octal sockets.
These days the EL509 is the tube that replaces the 6LF6 although not directly. It is a powerful sweep tube and is also used in OTLs. There is a non-plate cap variant that could be used, but FWIW the KT90 was developed from this tube, so if you really want something like that the KT90 is probably the place to look, as the design of the amp is made simpler by having no plate caps and the use of octal sockets.