Wikipedia has this to say about KT88s and their wattage;
"A KT88 push-pull pair in class AB1 fixed bias is capable of 100 watts of output with 2.5% total harmonic distortion or up to about 50W at low distortion in hi-fi applications."
Just because a tube distorts less obviously doesn't mean it's clean. That said, as I understand it you can get a clean 60 watts out of them. I only worked with tubes in video and announcing systems, I have never worked in tube amplification design, so my knowledge is finite. That's why I went to Don Sachs and Roy Mottram, I have learned a little bit more about them, but I am far from an expert.
"A KT88 push-pull pair in class AB1 fixed bias is capable of 100 watts of output with 2.5% total harmonic distortion or up to about 50W at low distortion in hi-fi applications."
Just because a tube distorts less obviously doesn't mean it's clean. That said, as I understand it you can get a clean 60 watts out of them. I only worked with tubes in video and announcing systems, I have never worked in tube amplification design, so my knowledge is finite. That's why I went to Don Sachs and Roy Mottram, I have learned a little bit more about them, but I am far from an expert.