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

That's probably why I kept my Quicksilver Mid Monos.    They ship with EL34 but can take any EL34 or 6L6 variant.    I've used KT 66,77,88 but I lean more toward the EL34 so I will put them in my system occasionally.

I have a 300b too and prefer that but sometimes I'll mix it up and put the QS in 

That's another reason I like those QS.   Sometimes I'm looking for a different presentation and will re tube with a 6l6.    My last quad was 6l6,  was just missing the el34 tone a little