Jond, FWIW, it's probably the the plate difference between the two amps which determine which tubes you can use. I don't know, but I'd risk a bet that the amp which can use EL34's has a plate voltage of 450 volts. The 500 volt unit would probably stress the EL 34's and cause them to fail. FWIW KT66's, 6L6gc' s and other 6l6 variants are not all equal, electrically speaking, so I wouldn't recommend that you use them without making sure that your amp would be appropriate for a specific tube.
KT-77 vs. KT-66
As I've had my QS Mono 60s, I've tried a variety of tubes. I have sold off my KT 150s, and use KT-88s from time to time, but what I really love are KT77's.
I'm thinking of getting some KT66s, thinking that lower powered tubes are better (to my ears) matches for my system. I just don't know what the KT-66's sound like.
Question: If you have compared KT77 and KT66 tubes, what would you say the sonic, audible characters of each are? Is there a difference you can hear? What does the KT66 sound like -- to you?