6550 vs KT88 vs KT90


From what I hear, these tubes are interchangable on most amps it seems that the 6550 is the least loved tube of the bunch. What are the sonic differences of these tubes? Thanks in advance for your reply.
jman
There is so much good information on this thread that I made a hard copy of it-- thanks everyone. Craig.
Many thanks to all on this thread and especially to Trelja who REALLY knows his tubes. This is what all the threads should be like. Mark me down as another happy KT88 owner. Anyone for driver tubes?
Trelja, just a note to you to mention that Jadis replied that the KT90s can be used in my old JA80s. I may get a set for when I run out of the NOS KT88s. Anyone with experience on how the new KT88s stack up against the older ones, sonically?
Rcprince, I am happy you put the question to Jadis. As stated before, they really seem to love the KT90. They go out of their way to recommend it in their products. I think Frank Garbie is a great guy, and seems to offer a lot of help. I have however, gotten information from him that conflicts with what Jadis advertises or says. Particularly in tube substitution. He wasn't aware my Orchestra Reference or some other Jadis amps accept tubes other than what they came with. He has given me excellent help when I have gone to him. I know horror stories abound, but I have nothing but positive things to say. I am sorry but I can not offer anything in regards to old KT88s, as I have no experience with them. I apologize. Enjoy the tubes, Krys!
I am trying to replace my tubes that originally was KT90's on my Jadis orchestra reference integrated amplifier. Beside that the KT90's are quite good I am curious of experiencing the KT88's specially from Valve Art, hoping to have little warmer sound with still good bass and extended treble of the 90's. If you have any experinence It would be great help. Thanks