I've run a lot of different KT88 and 6550 in two different amps.
They are all different but honestly not by a lot. Not enough to be worth changing tubes just for the sake of changing tubes the way a lot of guys do. That said, my favorite for its deep stage, liquid ease and lack of glare is the JJ KT88. The JJ is also one of the least expensive of all the KT88 and 6550 tubes I've tried.
Currently running the Tube Store Preffered Series KT88. One failed dramatically within a few weeks, and they are all noisy and not nearly as refined as the JJ.
They are all different but honestly not by a lot. Not enough to be worth changing tubes just for the sake of changing tubes the way a lot of guys do. That said, my favorite for its deep stage, liquid ease and lack of glare is the JJ KT88. The JJ is also one of the least expensive of all the KT88 and 6550 tubes I've tried.
Currently running the Tube Store Preffered Series KT88. One failed dramatically within a few weeks, and they are all noisy and not nearly as refined as the JJ.