6L6 vs EL34

Conversion is
changing 1 resistor in the bias circuit, changing screen grid resistors (if applicable), then bias it w a scope.
But you an change from EL34 ,6L6, KT66, 6550,etc. pretty easily.

EL34 being the European hi fi tube, specifically made for music.
6L6 being a home version of military (5881) amplifier tube.

But KT77 is already pretty sweet. I don't think there's going to be a huge change by swapping to EL34.
6L6: that's better for guitar amplifiers. Not really a hi fi tube, but Telefunken is selling them now for broadcast etc.anyway.

As an amplifier designer I can tell you that both 6l6 and el34 can be awesome. There are a  couple of things to consider. 1 el34 s need substantially more current for the heaters.  2 the bias is a little different.  3 el34 is a pentode while a 6l6 is a tetrode  connected pentode. In all my designs I make provisions for rolling both. Leaving the choice to the particulars of the client. Also the speaker driven. 6l6 dose it's finest with an oversized matching transformers and  cliptch or infinity.  While el34 is the thing to have with Polk audio or bose 901. I still recommend an oversized transformers just gives better bace and a more polished sound. 

I am a fan of the 6L6 tube, and a bigger fan of its cousin—the 350B (crazy expensive but good sounding).  Another similar sounding tube is the KT66.  

Listening to a 5881 amp now from Synthesis and really enjoying it, can take 6L6 tubes also.