KT 66 or NOS EL34

It will be different, but as for "better" that is hard to say. The change will depend on the EL-34 you choose and what you think of the sound. In very broad terms the sound should tighten up and emphasize the sound in the midrange and higher frequencies. The sound may become more detailed.
NOS EL-34s are really quite rare these days and very expensive. Which tube were you going to use?

My experience with those tubes in my Prima Luna 6 monos exactly mirrors that of the OP. The converse is that the EL34 is warmer sounding and more forgiving up top. I'd argue that, depending on your system's specific needs, you should choose the appropriate tube for that purpose.

Marty

I thought the EL34 "equivalent" was a KT77, not KT66. No?

Try the Genelex gold lion KT77. I tried a bunch of EL34's and 6ca7's in my VTL ST-85 and the KT-77 was way better. Never tried the KT-66
Alan

Arh did you try the RCA/Sylnania 6ca7 Fat boy tubes, i use them in my VTL ST 85.