Well everyone is right so far. Confused? You should be. The basics. The KT 90 IS the same tube as the GA KT99A, thought to be a wonderful tube. The 90 provides about 15% more output power than a KT 88(more headroom). It also seems to extend the bass down about an octave. It also sucks out some of the midrange and provides less highs than the 88. There also seems to be increased intermodulation distortion in the 90 but that could be a product of poor matching. Lastly, the 90's are generally bullit proof. They will handle a much higher plate current which means they can be biased higher. I have little experience with 6550 save to say that they have a weaker bass than the 88's and a purer midrange. A tube that you may be able to use that you have not mentioned is the E34L. If the plate current of your amp is low enough these tubes will give you much more of what you have gotten into tube audio for. Far better midrange bloom and life. A purer tone, the best sound hands down HOWEVER, it is likely that you will reduce the output power of your amps by about 30%. If you can try these you should. Also they are cheap.
