KT-77 vs. KT-66

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.

All things being equal, KT66=6L6 w kinkless filament.  KT77=EL34 kinkless filament.  That being said, I prefer the Gold Lion KT77 over my Tesla, Svetlana, RFT, VEB and Psvane (the worst) EL34s.  KT77 has more of everything of the EL34 sonic signature.  
 

i have Sylvania 6L6 GBs and GCs, and Tung Sol 5881s.  Different sonic signature than EL34, less bass but more airy twinkling treble.  I don’t have KT66 (yet!) to compare, but going off KT77, if you like the 6L6 sound in your amp, you’ll love the KT66.  I’ve had excellent luck with Gold Lion new production from EH.  
 

My tube rolling amp is a Dennis Had Inspire KT88, best $2,000 SEP on the planet, IMO. Very revealing of tube changes.

Cheers