With respect to its design, the KT-88 is the British variant of the 6550.
In use, they have proven less reliable than the 6550, particularly back in the 80's when their were some especially poor quality Chinese KT-88's that were more roman candle than output tube. VAC sells very good KT-88's.
It depends upon the use to which the tube is put and the production run, but in my experience, KT-88's tend to be a bit richer sounding than 6550's, while 6550's may be more accurate and linear.
In use, they have proven less reliable than the 6550, particularly back in the 80's when their were some especially poor quality Chinese KT-88's that were more roman candle than output tube. VAC sells very good KT-88's.
It depends upon the use to which the tube is put and the production run, but in my experience, KT-88's tend to be a bit richer sounding than 6550's, while 6550's may be more accurate and linear.