I've used KT66 in my Marshall VM2266. They're very dynamic and responsive to pick attack. Very different from any other EL34 or 6L6 tube I've used. I believe eurotubes characterizes them as being "very complex". They're cool but definitely in the 6L6 family (i.e. no crunch a la EL34). Not sure about using them in Mesa's. I've never heard of that yet.