About KT66 Tube
KT66 tube was introduced in 1938 in England. KT66 design was sold to RCA who turned it in to 6L6. KT66 has been used in high end audio amps in Europe, as well as in the legendary Marshall BluesBreaker combo amps in the 1960-70s.
KT66 vs 6L6
In general, KT66s tends to have a bit warmer sound with more low end. It is a good choice for those who are considering 6L6 upgrades. Please note that KT66 draws more heater current than 6L6. When in doubt, check with manufacturer or others who have used KT66 in 6L6 amps.
KT66 was originally developed for HiFi audio. Notably, it was used triode-connected in low distortion Williamson topology amps from Leak, Quad and Heath. These amps have excellent sonic which are still highly prized in classic Audiophile application. If you are considering 6L6 upgrade and your amp can handle filament current, KT66 is an excellent choice.