GEC in England designed KT66 as a better alternative to 6L6. KT66 is similar to 6L6 but has better linearity and lower distortion.
Many high-end audio amps used KT66 in Europe. Also, the legendary Marshall BluesBreaker combo amps in the 1960-70s used KT66.
In guitar amps, KT66s has a smooth bell-like sound with a good amount of low end. KT66 tube is not as scooped in the mids as the 6L6 tubes, which makes for a more all-round balanced tone.
KT66 was originally developed for HiFi audio. So it is no surprise that KT66 shines in high feedback, low distortion Williamson amps. KT66 amps, such as Quad 11, are still highly prized in the classic Audiophile market for this reason. KT66 is truly a great audio tube.
KT66 can be used in place of 6L6, but please note that KT66 draws more heater current than 6L6. When in doubt, check with the manufacturer or others who have used KT66 in 6L6 amps.