I believe the only thing it takes are EL84's, since they're a unique socket design compared to all the typical octal tubes (6L6, EL34, etc). The manual didn't really say anything about tube swapping in the RM20 (though it does contain the proper mv values for all the possible power tubes, I think this is because Randall "copied & pasted" a big chunk of the RM20 manual from some other MTS manual)
Too bad THD hasn't created a "reverse yellowjacket" that would allow 6L6's, EL34's, etc. to be used in EL84 amps. :idea: Of course, that may not be possible, but given my general level of "electronic ignorance" I'll just say that it'd be a cool idea.
--B