Are EL34 based amps more musical than KT series based ? Or it's purely a matter of design?

RM-9 wrings quite a bit out of EL-34 but Roger was a genius… Like Gordon Gow and the 6L6 MC240… but really no meaningful conversation is possible without also considering the massively important imo ….output transformers…. 

IF a VPI magic brick on the output xformer significantly alters the sound for the better, you know they skimped….. 

Just a thoughtful guess but given the changes in sound when switching the same kind of output tube by different manufacturers, the make of tube may have as much influence on the sound as the tube type.

Design. 

Something from the guitar amp world. EL 34s have a different distortion signature to 6L6s - an evolved KT66. 34s clip more quickly but have a smoother distortion - think of a Marshall stack v a Fender Twin Reverb.

However, we use Marshalls because we want them to distort. Not something we look for in a hi fi amp.