KT-77 vs. KT-66

as a generality, kt66/6l6 tubes are basically a weaker (less powerful) el34 -- they will usually sound weaker/slower in the bass, and sometimes a touch more ’beautiful’ in a classic tubey manner in the lower treble, mids and upper bass

but obviously, tube makes within a type, can have sonic differences that may overwhelm that across tube types (a gec kt77 vs a shuguang el34 may would sound more different than a modern russian sovtek kt66 for instance)

I'm not familiar with your amp.  I think your question tends to be amp specific.  In my Decware ZMA, I preferred the KT66 over the KT77.   however, steve Deckert designed the amp around the KT66.  I did a long journey of tube rolling and ended up right back where I started.  That said, my second favorite in this class was the inexpensive russian 6P3S.  Won't cost much to try it. 

And of course tube manufacturer plays a big role.

Jerry

KT66 and KT77 are both great tubes as @jjss49 states above the 66 will sound more classically tubey and a bit warmer but they sound great to my ears. Worth exploring since it's an option for you @hilde45 

Outstanding question since I have that question too. Im using the QS MidMono, so should be 20 watts lower than the 60. The 88' are slightly fatiguing....the 77' slightly less bass punchy, so I suppose the 66's would be even less punchy.

Suppose I'll stick with the 77's

Thanks

They’ll sound different in different circuits. I’m running GL KT66 in my Dyna/VTA 70s, and prefer them over any of the 6CA7s and EL34s I’ve tried. Midrange and lower treble are clear and lush. Bass had more extension with the Nesstone 6CA7, but the KT66 only feed the midbass and tweeter from 80hz+, where they do extremely well.