I had the same experience as Abe Collins regarding KT88 in my Conrad Johnson amps (11a, then 12's): The 6550 seemed a better match given the inherent slight sweetness of the amps. OTOH, my friends Sonic Frontiers amp (model 2 I think) benefited substantiall from the KT88.
I do think that KT88's are not all the same and that one brand, I forget if it's Svetlana or Sovtek, make KT88's that are particularly good for swapping into 6550 applications.
I do think that KT88's are not all the same and that one brand, I forget if it's Svetlana or Sovtek, make KT88's that are particularly good for swapping into 6550 applications.