FWIW, I believe that there is little, if any, difference between the KT99 and the KT90. The former's designation as a KT99 was adopted by the tube manufacturer (relabeler?) and sold at a premium price. So I was told. I used the KT99 in some Sonic Frontiers mono's. Nice tube/sound. I replaced them with 6550's and KT88's over the years I had them.
Best replacements for KT99A tubes?
I have a pair of Cary monoblocks with two broken KT99A tubes. As they are impossible to find, my plan is to replace them with a full set of new-manufacture tubes. I was looking at 6550's, but not set on anything yet. From reading various forums, I've identified the Gold Lions, Tung Sul, and Svetlana, but I haven't read anything that makes or breaks the decision for me. I'm looking for any and all input or suggestions! This is all very new to me, and I want to get something that will do justice to these very nice amps.